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Testing for normality 2 Duncan Golicher December 4, 2008 Consequences in practice Small samples We have seen that our ability to detect non-normality in a population using the data we collect in a sample will depend on how far the population deviates from normality and the size of our sample. The process of running a KS test or any other test of normality is no extra effort. The software will do all work for us. We know that with small sample sizes we are unlikely to reject the null. So what is the problem? Shouldn’t we just run the tests by default, then go ahead and carry out the analysis we planned, safe in the knowledge that we can defend ourselves against any accusation that the assumptions needed for the main analysis were not met by saying that we tested them formally. No. This is potentially really bad practice. The problem is this. If we run a KS test on a small sample we are likely to get a type two error on the test“for normality”. That’s another way of restating what we’ve just seen. As an exercise in reductio ad absurdum think of a sample size of two. We can conclude nothing about the distribution at all, so no test could ever reject the null hypothesis that it might be drawn from a normal population. This is clearly absurd and we would never run a test with such a small sample, but with any sample size below 20 the power of the test is still far too low to be really useful. By adopting the wrong statistical procedure as a result of an initial, apparently innocuous and merely technical type two error we are then more likely to get a type one error when we test the more important hypothesis we were really interested in. This is much more serious. We are likely to find significance when none exists. For example if by chance one very large value from a right skewed population is included when we take two smallish samples from a single population a KS test may suggest that there is nothing wrong with the assumption of normality. However a test on the hypothesis of no difference between samples may be erroneously significant. This is sometimes quite dangerous 1 . The result is not replicable and it could even get into the literature. If our desire is to conduct rigorous sound science we really should not be doing this. It defeats the object of running any statistical tests at all. Therefore if tests of simple hypotheses are required from very small samples non-parametric procedures are generally safer whatever the result of a the KS test. Large samples There is a much better case to be made for running tests of normality on large samples, providing we are aware of the true meaning of the test. In such situations the aim is not to evaluate whether a non-parametric test should be used. Instead we are interested in finding out how well any transformation of the data will work. Once a large data set has been assembled it is usually a real waste to use it only for simple non parametric null hypothesis testing. We want to know about effect sizes and produce confidence intervals. The most useful procedures nearly always involve some assumptions regarding distributions (models), although more advanced techniqes such as generalised linear models assume distributions other than normal. A well designed transformation effectively draws the population closer to a normal form. We can use a test of normality of the data both before and after transformation to help us evaluate how effective it has been. Inference about every aspect of the population is easier with large samples than small samples and this goes for the shape of the distribution too. What to do about it? So what are the answers in practice? Different authors will have slightly different takes on the problem, but if we follow these three steps we shouldn’t go that far wrong. 1 The fortunate outcome of including an extreme value is that it increases the variance and thus the standard error but this shouldn’t be taken as a guarantee that a type one error won’t result. 1

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Page 1: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Testing for normality 2

Duncan Golicher

December 4, 2008

Consequences in practice

Small samples

We have seen that our ability to detect non-normality in a population using the data we collect in a sample willdepend on how far the population deviates from normality and the size of our sample. The process of running aKS test or any other test of normality is no extra effort. The software will do all work for us. We know that withsmall sample sizes we are unlikely to reject the null. So what is the problem? Shouldn’t we just run the tests bydefault, then go ahead and carry out the analysis we planned, safe in the knowledge that we can defend ourselvesagainst any accusation that the assumptions needed for the main analysis were not met by saying that we testedthem formally. No. This is potentially really bad practice.

The problem is this. If we run a KS test on a small sample we are likely to get a type two error on the test “fornormality”. That’s another way of restating what we’ve just seen. As an exercise in reductio ad absurdum think ofa sample size of two. We can conclude nothing about the distribution at all, so no test could ever reject the nullhypothesis that it might be drawn from a normal population. This is clearly absurd and we would never run a testwith such a small sample, but with any sample size below 20 the power of the test is still far too low to be reallyuseful.

By adopting the wrong statistical procedure as a result of an initial, apparently innocuous and merely technicaltype two error we are then more likely to get a type one error when we test the more important hypothesis we werereally interested in. This is much more serious. We are likely to find significance when none exists. For example ifby chance one very large value from a right skewed population is included when we take two smallish samples froma single population a KS test may suggest that there is nothing wrong with the assumption of normality. Howevera test on the hypothesis of no difference between samples may be erroneously significant. This is sometimes quitedangerous1. The result is not replicable and it could even get into the literature. If our desire is to conductrigorous sound science we really should not be doing this. It defeats the object of running any statistical tests atall. Therefore if tests of simple hypotheses are required from very small samples non-parametric procedures aregenerally safer whatever the result of a the KS test.

Large samples

There is a much better case to be made for running tests of normality on large samples, providing we are aware ofthe true meaning of the test. In such situations the aim is not to evaluate whether a non-parametric test should beused. Instead we are interested in finding out how well any transformation of the data will work. Once a large dataset has been assembled it is usually a real waste to use it only for simple non parametric null hypothesis testing. Wewant to know about effect sizes and produce confidence intervals. The most useful procedures nearly always involvesome assumptions regarding distributions (models), although more advanced techniqes such as generalised linearmodels assume distributions other than normal. A well designed transformation effectively draws the populationcloser to a normal form. We can use a test of normality of the data both before and after transformation to helpus evaluate how effective it has been. Inference about every aspect of the population is easier with large samplesthan small samples and this goes for the shape of the distribution too.

What to do about it?

So what are the answers in practice? Different authors will have slightly different takes on the problem, but if wefollow these three steps we shouldn’t go that far wrong.

1The fortunate outcome of including an extreme value is that it increases the variance and thus the standard error but this shouldn’tbe taken as a guarantee that a type one error won’t result.

1

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1. Think about the properties of the underlying population first

2. Look for example data sets taken under similar circumstances

3. Simulate some data that follows from reasonable assumptions regarding the population and your samplingscheme multiple times.

Of the three steps the third is the least likely to be done in practice, but potentially the most informative.

Think about the population

Planning ahead is vital. We should never just collect data haphazardly assuming that the statistical analysis willadapt itself to whatever we happen to come up with. We can discount normality before we even look at any datain many situations

Samples consisting of counts

The fact that a population consists of discrete counts technically doesn’t allow it to take a perfectly normal form.However that is not necessarily much of a problem. It certainly doesn’t discount inference on the mean in a practicalsetting. In many cases it is the best way to handle the data. We intuitively understand the mean number of goalsper team per season, even though logically no team can score exactly 45.3 goals. Inference on the mean is veryuseful for many reasons. It provides us with confidence intervals and it is easy to communicate.

Statistical text books are full of details about normal approximations. If the mean value for counts is highenough (say above 7 as a pragmatic rule of thumb) we can safely assume that normal inference is OK providing wecan make another assumption. That is that the counts follow a Poisson distribution. In this case the population canbe expected to sufficiently symmetrical as to not interfere greatly with the process of inference. Poisson distributionsoccur when occurrences of an event are assigned randomly to classes and the number per class counted. This isa reasonable model for an underlying process in many cases and underlies statistical tests of association such aslog-linear modelling.

Poisson distribution with lambda=7

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Some textbooks suggest much larger values are needed2, but given that so many other things can go wrong inpractice a minor and fairly technical lack of symmetry should be the least of our worries for an empirical study.This is not to suggest that normal inference on this sort of data is always the best way to treat them, just that itgeneral no real harm will come of it.

On the other hand if the mean value for the counts is low, a population of counts can never be treated as normal.The problem is that zero forms a barrier that automatically skews the data.

2True symetry starts at about lambda=10 and at lambda =30 the normal approximation is really very good.

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In ecology there are often far worse problems. They are so common that they should be expected as the defaultwhen we count most types of organisms. They arise from clustering effects. Animals and plants tend to sticktogether, so the assumption of random allocation of individuals is hardly ever met. Take the example of bats whichroost in house lofts. Most houses will have no individuals at all, but some houses will have a colony. The populationmight then look something like this.

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This is a zero inflated poisson. In this sort of case one solution is to separate data analysis in two phases. Firstwe can look at presence and absence as a binary variable and perhaps analyse the reasons for absence. Then wemight look at the number of individuals in the colonies themselves. For something like bats it is quite logical tothink in terms of mean colony size before starting work, but in other cases it may be less obvious that this approachis going to be needed. Sometimes the zeros get mixed in and analysed along with the rest of the data, even thoughthey are really part of a different process. As the mean value for the poisson part is reduced the distribution becomesa mixture. This can really make a mess of the analysis. In some cases if a mean group size is not going to tell usmuch it might be easier to only measure presence and absence.

It is also very common to find a particularly nasty sort of skewed poisson distribution known as the negativebinomial in ecological data. This occurs in cases when there are a few very large agglomerations of individuals andmany small clusters. Flocking birds, such as starlings, can have this sort of pattern, particularly when viewed overthe whole year. Much of the time they are in pairs or small family groups, but in the winter they come together inthe thousands. This is perhaps the hardest sort of data to handle consistently well because if one large cluster ismissed inference can be way out. Hilborn and Mangel give an example of bycatch of allbatross in the South Pacificas an illustration. Most of the time very few birds are caught, but occasionally a boat kills a lot of birds. If thiswere to be missed we might not take adequate conservation measures.

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In all these cases the way to proceed in the case of a real life study is to anticipate the problem beforehand,think carefully and take advice from ecologists with experience working with the organism in question. There aresolutions in all cases, but many require alterations to the study design. No statistical method has ever managedto deal effectively with a very small sample taken from a highly skewed population. Mess up in the planning stageand there is no way back.

Log normal data

Many phenomena in nature are more likely to produce log-normal distributions, or variations on the theme, thannormal distributions. When many effects act additively a normal distribution is expected due to the mechanism ofthe central limit theorem. However when effects act in a multiplicative fashion a log-normal results for a similarreason. Exponential growth or decline is such an effect. Log normal distributions are right skewed. So for examplethe populations of towns often form a log-normal type pattern as do the populations of species. Wealth accumulationalso tends to be log-normal unless governments intervene. It is not so much that the rich get rich and the poorget poorer, rather that the rich get richer more quickly. Dividing up land produces a log normal for the conversereason. If we first split a parcel of land in two and then split one of the resulting parcels in two, then we get avariety of exponential decay. Thus the area of land holdings are often log normal too.

So we end up with situations where we find that most towns are small, but most people live in large towns.More species are rare, but most individuals belong to a a handful of species. Most people do not have much money,but most of the money is in a few people’s bank accounts. Most people have very small parcels of land, but mostof the land belongs to one owner. And so on.

The good thing about a genuine log normal is that simply taking logarithmns makes it normal. So we have adead easy get out if we know beforehand that a log normal effect is likely to occur. In fact taking logs unecessarilyoften does no great harm. A truly normal population will then become left skewed but in real life the outliers areusually on the right so it will tend to draw them in. we can even take logs twice to correct for extreme skew. Themain problem with this may be communication and interpretation of the results.

A log normal population

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Taking logs of a log−normal

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Concentrations

The third main class of non normal populations consist of measurements of concentrations. Unlike the log-normalcase we have no real theory to guide us in this, we have to use intuition and common sense. The problem is suggestedby the very term concentration. Pollutants and contaminants such as heavy metals are often concentrated inspecific areas or in a few individuals or organs within individuals. Thus we might get many soil “samples”3 withvery low levels followed by one taken right next to a discarded battery. The same effect can occur in water samplesor measurements taken from fish such as salmon.

When looking at contaminants it might often be worth asking whether inference on the mean makes any senseat all. The problem of pollutants is precisely that they are concentrated. In most cases there are regulations andguidelines that set “safe” limits for acceptable concentrations. These may be arbitrary but we will often find thatmost samples are well below them but a few are clearly way above. Thus the data can be converted into a binaryvariable (“safe”,”unsafe”) and analysed as such. Or several categories such as normal, slightly contaminated, highlycontaminated can be extracted from the literature. we might not die from eating a plate of shellfish all with themean amount of toxins, but if we get one really bad one that we’ve had it.

If we do actually want to look at mean values the central limit theorem can perhaps come to our aid. Forexample when soil is collected several “samples” taken from a site are often mixed to form a combined “sample”before analysis. Notice that is conventional to call a soil core a soil sample, but in a statistical sense it is not.It is one unit that makes up the statistical sample. The good thing about this procedure is that adding severalnumbers together from almost any form of distribution tends to produce a normal distribution in the end. Howeverif the population is highly skewed that still is unlikely to really solve the problem unless a very large number ofsub-samples are mixed together.

> concentrations <- c(1, 1, 2, 3, 4, 5, 6, 2, 8, 2, 12, 4, 20,

+ 10, 40)

> hist(replicate(1000, sum(sample(concentrations, 10, replace = T))),

+ main = "", xlab = "Pooled soil samples", col = "grey")

3It is very difficult not to use the word sample in this context even though it referes to a sample unit, not a statistical sample

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A further point regarding concentrations is also that they will tend to be concentrated in time and space. Itis this effect that we are often interested in understanding. However this can lead to autocorrelation in the datawhich can potentially play havoc on statistical inference.

Use other studies

It is suprising how often studies are planned only with reference to the summarised findings of previous similarwork. This is particularly unfortunate if the summaries are only based on null hypothesis tests with no informationregarding effect size and confidence intervals. It is fortunately now becoming common for authors to provide rawdata in appendices on line. If this is available it should always be used as it can provide vital insight into the sortof distribution to be expected.

The subject of the use of prior experience is a large one and should be expanded upon.

Simulate some data from a condidate population

Our intuitions regarding the way sampling affects the patterns we see when looking at histograms, boxplots, qqplotsand ecdfs are often not reliable. A“hole” in a histogram might be an indication of bimodality, but with small samplesit may arise by chance. Even quite large samples never look perfectly normal. This is the justification for usingtests of normality in the first place. If only they worked the way we really want the world would be a much simplerplace.

I have provided a set of simulated results to give some idea as to the patterns to expect. This exercise is muchmore informative when you conduct it yourself using the same design as will be used for the survey or experiment.Histograms are often the most difficult to interpret well and are susceptible to decisions regarding the number ofbins used for the data. Most software takes the decision for you, but it is usually a good idea to try with variousbin sizes.

The idea behind including the appendix that follows is to allow you to get a quick visual impression of thevariability in the results that might be expected when you conduct a standard exploratory analysis on a sampletaken from a known population. In all cases the statistical inference is aimed at the underlying population (that isknown) not the sample.

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Appendix. Simulated data

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−1.5 −0.5 0.5 1.5

78

910

1112

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

910

1214

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

67

89

11

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−1.5 −0.5 0.5 1.5

89

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

89

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

●●

−1.5 −0.5 0.5 1.5

67

89

1012

Normal Q−Q Plot

Theoretical Quantiles

● ●

●●

−1.5 −0.5 0.5 1.5

78

911

Normal Q−Q Plot

Theoretical Quantiles

●68

1012

14

Normal Q−Q Plot

●●

−1.5 −0.5 0.5 1.5

910

1112

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

910

1112

13

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

● ●

89

1011

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

67

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

89

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

46

810

12

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

−1.5 −0.5 0.5 1.5

910

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

89

1113

15

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

67

89

11

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

7

Page 8: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Right skewed log normal

x

Fre

quen

cy

0 20 40 60 80 100 120

050

100

150

200

250

300

Histograms

x

0 10 20 30 40

01

23

45

x

0 5 10 20 30

0.0

1.0

2.0

3.0

x

0 20 40 60 80

01

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60

12

34

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01

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01

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0.0

0.5

1.0

1.5

2.0

x

0 10 20 30 40

01

23

4

x

0 10 30 50

01

23

45

67

x

5 10 15 20 25 30

01

23

45

60.

01.

02.

03.

0

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0 5 10 20 300

12

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6

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0 10 20 30 40 500

12

34

5

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0.0

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2.0

3.0

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0 5 10 15 20 25

01

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0 5 10 15 20 25

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0 5 10 20 30

0.0

1.0

2.0

3.0

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0 5 10 15 20 25

01

23

45

x

Fre

quen

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5 10 15 20 25 30

0.0

1.0

2.0

3.0

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0 5 15 25 35

01

23

45

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quen

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01

23

4

x

Fre

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0 10 20 30 40 50

01

23

45

x

Fre

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0 20 40 60

01

23

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x

Fre

quen

cy

0 5 10 15 20 25

01

23

4

Fre

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01

23

4

x

Fre

quen

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5 10 20 30

0.0

1.0

2.0

3.0

x

Fre

quen

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0 5 10 20 30

0.0

1.0

2.0

3.0

x

Fre

quen

cy

0 5 10 15 20 25

0.0

1.0

2.0

3.0

Fre

quen

cy

01

23

45

Boxplots

510

2030

510

1520

255

1015

20

515

2535

510

1520

2530

510

1520

25

1020

3040

510

1520

1020

3040

510

1520

25

1020

3040

50

1020

3040

46

812

165

1015

2025

510

1520

2510

2030

40

1020

3040

46

812

165

1015

2025

306

812

16

510

1520

510

1520

250

1020

3040

50

515

25

510

1520

510

1520

255

1015

2025

510

1520

020

4060

46

812

16

1020

3040

1020

3040

510

15

510

2030

510

1520

1020

3040

510

1520

68

1216

510

1520

255

1015

20

010

2030

405

1015

20

515

2535

46

810

14

510

1520

510

1520

2530

510

1520

256

810

1214

Qqplots

●●

−1.5 −0.5 0.5 1.5

510

1520

Normal Q−Q Plot

Theoretical Quantiles

−1.5 −0.5 0.5 1.5

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

●●

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

●●

510

1520

Normal Q−Q Plot

−1.5 −0.5 0.5 1.5

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

510

15

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

510

1520

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

510

1520

25

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

−1.5 −0.5 0.5 1.5

510

1520

2530

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

−1.5 −0.5 0.5 1.5

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

● ●●●

−1.5 −0.5 0.5 1.5

1030

5070

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

510

1520

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−1.5 −0.5 0.5 1.5

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●●

●●

−1.5 −0.5 0.5 1.5

46

812

16

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

1020

3040

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

● ●●

●●

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

●●

−1.5 −0.5 0.5 1.5

510

15

Normal Q−Q Plot

Theoretical Quantiles

05

1525

Normal Q−Q Plot

−1.5 −0.5 0.5 1.5

46

810

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

● ●

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

510

1520

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

46

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

510

1520

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

1020

3040

50

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

1030

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

● ●

●●

−1.5 −0.5 0.5 1.5

020

4060

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

1015

2025

30

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

● ●

−1.5 −0.5 0.5 1.5

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

●●

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

−1.5 −0.5 0.5 1.5

510

1520

Normal Q−Q Plot

Theoretical Quantiles

510

1520

Normal Q−Q Plot

● ●

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

515

2535

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

● ●

−1.5 −0.5 0.5 1.5

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

510

1520

25

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

68

1216

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−1.5 −0.5 0.5 1.5

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●● ●

−1.5 −0.5 0.5 1.5

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

1015

20

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

8

Page 9: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Sample size 20

Normal

x

Fre

quen

cy

5 10 15

050

010

0015

0020

00

Histograms

x

8 9 10 12 14

01

23

45

67

x

4 6 8 10 12 14

01

23

45

x

4 6 8 10 14

02

46

80

12

34

x

Fre

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4 6 8 10 14

02

46

8

x

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6 8 10 12 14 16

02

46

810

x

Fre

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6 8 10 12 14 16

01

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67

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6 8 10 12 14

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6 7 8 9 11 13

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6 8 10 12 14

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67

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6 8 10 12 14

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6 7 8 9 11 13

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67

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7 8 9 10 11 12

01

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67

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6 8 10 12 14

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6 8 10 12 14

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67

02

46

810

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6 8 10 12 14

01

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6 8 10 12

01

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6 8 10 12 14

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6 8 10 12 14

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6 8 10 12 14 16

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7 8 9 10 12

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6 8 10 12 14

01

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6 8 10 12 14

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67

Fre

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01

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45

Boxplots

68

1012

146

810

124

68

1014

67

89

11

68

1012

14

68

1012

810

1214

810

1214

68

1012

78

911

13

911

1315

68

1012

●68

1012

810

1214

810

1214

68

1012

14

78

911

137

89

1011

810

1214

810

1214

78

911

137

89

1113

78

911

137

89

1113

68

1012

89

1113

810

1214

67

89

1113

89

1011

126

810

126

810

128

910

12

89

1012

78

911

138

1012

14

89

1113

68

1012

89

1011

128

910

1112

13

810

1214

68

1012

78

911

136

810

128

910

12

46

810

126

810

1214

68

1012

148

1012

14

Qqplots

●●

●●

●●

−2 −1 0 1 2

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

−2 −1 0 1 2

46

810

12

Normal Q−Q Plot

Theoretical Quantiles

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

●●

●● ●

●●

68

1012

Normal Q−Q Plot

●●

−2 −1 0 1 2

78

910

1112

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

−2 −1 0 1 2

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

78

910

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●●

● ●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●●

−2 −1 0 1 2

89

1011

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

89

1113

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

−2 −1 0 1 2

78

910

12

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

−2 −1 0 1 2

78

910

12

Normal Q−Q Plot

Theoretical Quantiles●

●●

●●

68

1012

Normal Q−Q Plot

●●

●●

●●

−2 −1 0 1 2

89

1011

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

89

1011

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

89

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

89

1012

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

78

910

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

89

1012

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles●

●●

●●

−2 −1 0 1 2

67

89

1113

Normal Q−Q Plot

Theoretical Quantiles

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

●●

810

1214

Normal Q−Q Plot

●●●

●●●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

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−2 −1 0 1 2

67

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

68

1014

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●●

−2 −1 0 1 2

68

1014

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●

68

1012

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

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●●

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

67

89

11

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

9

Page 10: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Right skewed log-normal

x

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0 20 40 60 80 100

020

4060

8010

012

014

0

Histograms

x

0 5 15 25 35

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46

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x

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12

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24

68

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810

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0 5 15 25 35

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6

Boxplots

510

1520

255

1015

2025

30

510

1520

515

25

515

2535

515

2535

510

1520

25

1020

3040

510

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1020

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510

1520

25

510

2030

510

1520

515

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510

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510

2030

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812

165

1015

2025

510

1520

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510

1520

515

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1020

3040

510

1520

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515

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1020

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515

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3040

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515

2535

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4060

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510

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●●

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100

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010

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24

68

1216

1020

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50

1020

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515

2535

Qqplots

●●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles●

●●

●●

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●●

−2 −1 0 1 2

510

1520

Normal Q−Q Plot

Theoretical Quantiles

● ●●

●● ●

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−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

●●

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1020

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Normal Q−Q Plot

●●

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−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●●

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●●

−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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515

2535

Normal Q−Q Plot

Sam

ple

Qua

ntile

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●●

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−2 −1 0 1 2

010

2030

4050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

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●●

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−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

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Qua

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●●

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●●

1020

3040

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

● ●●

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−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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−2 −1 0 1 2

510

1520

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●●

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●●●●

●●

−2 −1 0 1 2

2040

6080

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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1020

3040

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●● ●

●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

●●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

● ●●

●●

●●

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−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

●●

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1020

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Normal Q−Q Plot

●●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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●●

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−2 −1 0 1 2

05

1020

30

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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2040

6080

Normal Q−Q Plot

Sam

ple

Qua

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−2 −1 0 1 2

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

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Qua

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●●

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−2 −1 0 1 2

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●● ●

●●

●●

● ●●

−2 −1 0 1 2

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●●

●●

●●

●●

●510

2030

Normal Q−Q Plot

Sam

ple

Qua

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●● ●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

510

15

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

510

1520

25

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

●●

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−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles●

●● ●

●●

●●

1020

3040

Normal Q−Q Plot

●●

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●●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

●●●

● ●

●●

−2 −1 0 1 2

1030

5070

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●●●

●● ●●

● ●

2040

6080

Normal Q−Q Plot

Sam

ple

Qua

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s

●●●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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●●●

● ●

●●

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−2 −1 0 1 2

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

515

2535

Normal Q−Q Plot

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●●

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−2 −1 0 1 2

1020

3040

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●●

●●

●●

●●

1020

3040

Normal Q−Q Plot

Sam

ple

Qua

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10

Page 11: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Sample size 30

Normal

x

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5 10 15

050

010

0015

0020

00

Histograms

x

6 8 10 12 14

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x

8 10 12 14

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Boxplots

68

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126

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13

Qqplots●

●●

●●

● ●●

●●

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−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

●●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

68

1012

14

Normal Q−Q Plot

●●

●●

−2 −1 0 1 2

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

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●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

−2 −1 0 1 2

78

910

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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●●

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−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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●●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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●●

●●

●●

●●

46

810

12

Normal Q−Q Plot

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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●●

−2 −1 0 1 2

46

810

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

68

1012

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

●●● ●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

−2 −1 0 1 2

89

1012

Normal Q−Q Plot

Theoretical Quantiles●

● ●

●●

●●

●●

68

1012

Normal Q−Q Plot

●●

●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●●

−2 −1 0 1 2

78

911

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

810

1214

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

78

911

13

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

●●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

910

1112

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

89

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

46

810

1214

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

810

1214

16

Normal Q−Q Plot

Theoretical Quantiles

●●

●●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles●

●●

●●

●●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

●●

● ●

●●

● ●

●●●

●●

89

1113

Normal Q−Q Plot

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

67

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

78

911

13

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

● ●●

●●

●●

−2 −1 0 1 2

67

89

11

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

89

1011

12

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●

−2 −1 0 1 2

89

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

78

910

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

810

1214

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

11

Page 12: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Right skewed log-normal

x

Fre

quen

cy

0 20 40 60 80

020

4060

8010

012

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0

Histograms

x

0 5 15 25 35

02

46

812

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05

1015

x

0 10 20 30 40 50

02

46

812

02

46

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x

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810

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Fre

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1015

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05

1015

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Fre

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0 10 20 30 40

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46

810

x

Fre

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0 10 20 30 40

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812

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0 20 40 60 80

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1015

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Fre

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0 10 20 30 40 50

05

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0 10 20 30 40

02

46

810

Fre

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46

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x

0 5 10 20 30

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46

810

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0 10 30 50

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0 5 10 20 30

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68

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68

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Fre

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0 10 20 30 40 50

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46

810

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x

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0 10 30 50

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812

x

Fre

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68

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Fre

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Fre

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02

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810

x

Fre

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0 20 40 60

05

1015

x

Fre

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0 5 10 20 30

02

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810

x

Fre

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0 10 20 30 400

24

68

10

Fre

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02

46

812

x

0 20 40 60 80

05

1015

x

0 5 10 20 30

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812

x

0 5 10 15 20 25

02

46

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510

15

x

Fre

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0 10 20 30 40

02

46

8

x

Fre

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0 10 20 30 40

02

46

8

x

Fre

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0 50 100 150

05

1015

20

Fre

quen

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02

46

810

x

Fre

quen

cy

0 20 40 60

05

1015

x

Fre

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0 10 20 30 40

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46

8

x

Fre

quen

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0 10 20 30 40

02

46

8

Fre

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02

46

8

x

Fre

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0 10 20 30 40

02

46

810

x

Fre

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0 10 20 30 40 50

05

1015

x

Fre

quen

cy

0 5 15 25 35

02

46

8

Fre

quen

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02

46

810

Boxplots●

1020

3040

1020

3040

1020

3040

50

05

1525

35

1030

50

010

3050

1030

50

1020

3040

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515

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1020

3040

510

2030

510

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●●

1020

3040

1020

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010

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1525

1030

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1030

50

020

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●●

510

2030

1020

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1030

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●●

1020

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2030

510

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020

4060

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05

1020

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●●

1020

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●●

010

2030

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1020

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1020

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510

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510

1520

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020

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●●

510

2030

515

2535

1020

3040

510

1520

250

1020

3040

50

1030

50

1030

50

●●

510

2030

1020

3040

Qqplots

●●

●●

●●

●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles●

●●

●●

●●

●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

● ●●

●● ● ●

●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

●●

●●

●●

1020

3040

Normal Q−Q Plot

●●

●●

●●●

●●

−2 −1 0 1 2

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●● ●

●●●

●●

●●

●●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●●

●●●●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

515

2535

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

010

3050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●●

●●

●●

−2 −1 0 1 2

020

4060

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

● ●

●●

●●

●●

●515

2535

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

● ●

●●

● ●

● ●

●●

●●

●●

−2 −1 0 1 2

05

1525

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

010

2030

40

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●● ●●

−2 −1 0 1 2

010

2030

4050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

1020

3040

50

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

● ●● ●

●●

●●

−2 −1 0 1 2

1020

30

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●●

●●

−2 −1 0 1 2

010

2030

40

Normal Q−Q Plot

Theoretical Quantiles

●●●

●●

●●

●●

●●

●●

●●●●

● ●

−2 −1 0 1 2

2040

6080

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

●●

●●

1030

50

Normal Q−Q Plot

●●

●● ●

●●

−2 −1 0 1 2

05

1525

35

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

● ●

−2 −1 0 1 2

010

3050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●●● ●

●●

●●●●●●

●●●

●●●

−2 −1 0 1 2

1030

5070

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

515

2535

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●●●

● ●

● ●●

●●

●●

−2 −1 0 1 2

1030

5070

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

● ●

●●●●

●●

●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●

●●● ●

510

2030

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●● ●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●●

●●

●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●●

● ●

●●●

●020

4060

80

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

510

2030

Normal Q−Q Plot

Theoretical Quantiles

●●

●●●

●●

−2 −1 0 1 2

510

1520

25

Normal Q−Q Plot

Theoretical Quantiles

●●

●● ●

● ●

●●

●●

●●

●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

●●

●●●●

●●

●●●

●●

●●

● ●●

●●

1030

50

Normal Q−Q Plot

●●●

●●●●●

●●

●●

●●

−2 −1 0 1 2

010

2030

4050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

●●●

−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

● ●

●●

●1020

3040

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●●

●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●●

●●● ●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

● ●

●●●

●●●

●●

●●

●●

−2 −1 0 1 2

020

4060

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●10

2030

4050

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●●

●●●

●●

●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●●● ●

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

010

3050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

515

2535

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

12

Page 13: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Sample size 50

Normal

x

Fre

quen

cy

5 10 15

050

010

0015

0020

00

Histograms

x

6 8 10 12 14

02

46

810

x

6 8 10 12 14

05

1015

x

6 8 10 14

02

46

812

05

1015

x

Fre

quen

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4 6 8 12 16

05

1015

20

x

Fre

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4 6 8 10 14

02

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812

x

Fre

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6 8 10 12 14

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46

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Fre

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05

1015

x

Fre

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6 8 10 12 14 16

02

46

812

x

Fre

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cy

6 8 10 12 14

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46

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x

Fre

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4 6 8 10 14

05

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02

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x

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4 6 8 10 14

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20

x

Fre

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6 8 10 12 14

02

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810

x

Fre

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4 6 8 10 14

05

1015

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812

x

6 8 10 12 14

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810

x

4 6 8 10 12 14

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46

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x

4 6 8 10 14

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x

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Boxplots●

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●●

68

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810

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166

810

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810

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810

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68

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910

126

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68

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46

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68

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68

1012

14

Qqplots

●●

●●

●●

●●

●●

●●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●●●

●78

911

13

Normal Q−Q Plot

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

810

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●●

●●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

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●●

●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

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●●●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●● ●

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

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68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●●

●●●

●●

●●

●●

−2 −1 0 1 2

68

1014

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

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●●

●●

●●

●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

46

810

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

●●

●●

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46

810

1214

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

● ●

●●●●

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●●

●●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

●●●●

●●

●●

●●

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−2 −1 0 1 2

46

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●

●●●

●●

●●

●●●

●●●

−2 −1 0 1 2

810

1214

16

Normal Q−Q Plot

Theoretical Quantiles

●●

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68

1012

14

Normal Q−Q Plot

●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

46

810

12

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

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●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●●

●●

●●

●●

●●

−2 −1 0 1 2

68

1012

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

●●

●●

●●

46

810

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●●

●●●

−2 −1 0 1 2

46

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●

● ●

●●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

●●

●●

●●

●●

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●●

68

1012

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

●●

●●●

●●

●●

● ●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

●●

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68

1012

1416

Normal Q−Q Plot

●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

●●

●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

● ●

●●

● ●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

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●●

●●

●●

●●

●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

46

810

1214

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

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● ●

●●

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−2 −1 0 1 2

67

89

1113

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

68

1012

14

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

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●●

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−2 −1 0 1 2

46

810

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

●●

●●

●●●

●●

●●

−2 −1 0 1 2

68

1012

14

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

68

1014

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

13

Page 14: Testing for normality 2 - Duncan Golicher's weblog · PDF fileTesting for normality 2 Duncan Golicher December 4, 2008 ... We can use a test of normality of the data both before and

Right skewed log-normal

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Qqplots

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−2 −1 0 1 2

1020

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Normal Q−Q Plot

Theoretical Quantiles

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● ●

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−2 −1 0 1 2

1030

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Normal Q−Q Plot

Theoretical Quantiles

●●

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●● ●●

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−2 −1 0 1 2

010

2030

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Normal Q−Q Plot

Theoretical Quantiles

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010

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Normal Q−Q Plot

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−2 −1 0 1 2

515

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

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−2 −1 0 1 2

1030

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

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−2 −1 0 1 2

020

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

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515

2535

Normal Q−Q Plot

Sam

ple

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●●●

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●●●● ●

−2 −1 0 1 2

020

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

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s

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●●

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● ●● ●

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−2 −1 0 1 2

1030

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

1020

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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515

2535

Normal Q−Q Plot

Sam

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−2 −1 0 1 2

020

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

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s

●●

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−2 −1 0 1 2

1020

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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−2 −1 0 1 2

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●●

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Normal Q−Q Plot

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−2 −1 0 1 2

020

6010

0

Normal Q−Q Plot

Theoretical Quantiles

●●

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● ●●

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●●●●

●●

●●

−2 −1 0 1 2

010

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Normal Q−Q Plot

Theoretical Quantiles

●●

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●●

●●●

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●●●

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●●

●●

−2 −1 0 1 2

1030

5070

Normal Q−Q Plot

Theoretical Quantiles

●●

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●●

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● ●

010

2030

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Normal Q−Q Plot

●●

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−2 −1 0 1 2

2040

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

●●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

●●

● ●●●

● ●

●●

●●

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−2 −1 0 1 2

1030

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●● ●

●●

●●

●●

●●

●●

●●

●1020

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Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

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●●●

● ●●

●●

●●

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●●●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●●

●●

●●

●●

●●

●●

●●

●●●

−2 −1 0 1 2

1020

30

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●●●

●●

● ●

●●

●●

●●●

●●

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−2 −1 0 1 2

1020

3040

50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

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●●

●●

●●

1020

3040

Normal Q−Q Plot

Sam

ple

Qua

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s

●●

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●●

●●

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−2 −1 0 1 2

010

2030

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●●

●●

●●

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●●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

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●●

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−2 −1 0 1 2

1020

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50

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●●

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● ●●

●●

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1020

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Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●

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●●

●●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

●●

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●●

●●●

●●

●●

●●

●● ●

●●

●●

●●

−2 −1 0 1 2

010

3050

Normal Q−Q Plot

Theoretical Quantiles

●●●●●

●●

●●

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●●

−2 −1 0 1 2

515

25

Normal Q−Q Plot

Theoretical Quantiles

●●

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010

3050

Normal Q−Q Plot

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−2 −1 0 1 2

1020

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Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●

●●

●●

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

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●●

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●●

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●●●●

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● ●

● ●

020

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Normal Q−Q Plot

Sam

ple

Qua

ntile

s

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−2 −1 0 1 2

515

2535

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

● ●

●●

●●

●●●

●●

●●

●●●

●●

●●

−2 −1 0 1 2

010

3050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

● ●●

●●●

●●

●●

●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

●●

●●

●●

05

1020

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

●●●

●●

●●

●●

●●

●●●●●

●●●

●●●

−2 −1 0 1 2

1020

3040

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●

●●

●●

●●

●●●

●●●

−2 −1 0 1 2

010

3050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●

●●

●●●●

●●

●●

●●

●●

●●●●

●●●

●●

●●

●●

●●

●●

−2 −1 0 1 2

010

2030

4050

Normal Q−Q Plot

Theoretical Quantiles

Sam

ple

Qua

ntile

s

●●●

●●●●●

●●

●●

●●●

●●

●●

●●

●●

1020

3040

50

Normal Q−Q Plot

Sam

ple

Qua

ntile

s

References

Hilborn, R. and Mangel, M. (1997). The Ecological Detective: Confronting Models with Data.

14