Univariate Statistics Summary - learn.stleonards.vic.edu.au · Univariate Statistics Summary Page 1 of 21 Further Maths Univariate Statistics Summary Types of Data Data can be classified

Univariate Statistics Summary

Page 1 of 21

Further Maths Univariate Statistics Summary

Types of Data Data can be classified as categorical or numerical.

Categorical data are observations or records that are arranged according to category. For example:

the favourite colour of a class of students; the mode of transport that each student uses to get to

school; the rating of a TV program, either “a great program”, “average program” or “poor program”.

Postal codes such as “3011”, “3015” etc.

Numerical data are observations based on counting or measurement. Calculations can be performed

on numerical data.

There are two main types of numerical data

Discrete data, which takes only fixed values, usually whole numbers. Discrete data often arises as

the result of counting items. For example: the number of siblings each student has, the number of

pets a set of randomly chosen people have or the number of passengers in cars that pass an

intersection.

Continuous data can take any value in a given range. It is usually a measurement. For example: the

weights of students in a class. The weight of each student could be measured to the nearest tenth of

a kg. Weights of 84.8kg and 67.5kg would be recorded. Other examples of continuous data include

the time taken to complete a task or the heights of a group of people.

Exercise 1

Decide whether the following data is categorical or numerical. If numerical decide if the data is

discrete or continuous.

1.

2.


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3.

4.

Solutions

1a Numerical-discrete b. Categorical c. Categorical

d. Numerical-Continuous

2a. Numerical b. Categorical c. Numerical d. Categorical e. Categorical

3a Continuous b. Discrete c. Continuous d. Continuous e. Discrete

4. D

Representing Data Data is often represented in frequency charts, column charts, histograms and dot plots.

Remember: the frequency of an observation is the number of times that observation occurs.


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Example 1:

The following frequency distribution table gives the number of days of each weather type for the

month of January. Represent the information using a column chart.

Example 2

Represent the data in

a. a histogram. b. a frequency polygon

a.

b.

Notice that for a histogram there is no gap between the bars and the number of visits are positioned

at the centre of each bar.

0

5

10

15

20

Hot Warm Mild Cool

Fre

qu

en

cy

Weather

Weather Type


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Example 3

Represent the data in a. a histogram b. a frequency polygon

a.

b.

Notice that the numbers are placed at the edges of the bars along the x-axis for grouped data.

Types of Average There are three types of average which can represent a set of data. An average is a measure of

central tendency.

The Mean

The most common average is the mean.

�̅� is used to denote the mean.

�̅� = 𝑠𝑢𝑚 𝑜𝑓 𝑎𝑙𝑙 𝑠𝑐𝑜𝑟𝑒𝑠

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑐𝑜𝑟𝑒𝑠

Example 1: The following data gives the number of pets kept in each of 10 different households.

3, 5, 4, 4, 2, 3, 0, 1, 4, 5

The mean number of pets is given by:

3 + 5 + 4 + 4 + 2 + 3 + 0 + 1 + 4 + 5

10= 3.1

The mean is sometimes not the best average to use as it is affected by extreme scores or outliers.


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The Median

The median of a set of scores is the middle score when the data are arranged in order of size.

The median’s position is given by 𝑛+1

2th score, where n is the number of scores.

In example 1 the median’s position is given by the 10+1

2 th score. This is the 5.5th score or halfway

between the 5th and 6th score, after the scores have been arranged in order of size.

Arranging the data in order of size:

0, 1, 2, 3, 3, 4, 4, 4, 5, 5

Median number of pets is: 3+4

2= 3.5 (as there are two middle scores we take their mean.

The median is not affected by extreme values or outliers.

The Mode

The mode of a group of scores is the score that occurs most often. That is the score with the highest

frequency.

In example 1 the modal number of pets is 4. More than one mode is possible.

Frequency Tables

Example 4

The table indicates that 6 students made 0 cinema visits, 7 students made 1 cinema visit, 4 students

made 2 cinema visits etc.

The mean number of visits can be found by adding an extra column to the table and multiplying the

number of visits by the frequency.

Number of visits (𝒙) Frequency (𝒇) 𝒇 × 𝒙

0 6 0

1 7 7

2 4 8

3 2 6

4 1 4

Total 20 25

a. mean number of visits =𝑡𝑜𝑡𝑎𝑙 𝑜𝑓 𝑓×𝑥

𝑡𝑜𝑡𝑎𝑙 𝑜𝑓 𝑓=

25

20= 1.25


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b. the median number of visits can be found by finding the position of the median as the number of

visits are in order of size in the table.

The median’s position is the 𝑛+1

2th score =

20+1

2th = 10.5th position. Halfway between the 10th

and 11th scores. The median’s position falls within the second row and is therefore 1.

c. The mode is the score with the highest frequency. The mode is 1.

Alternatively, the mean and median can be found using Lists and Spreadsheet in the calculator.

1: Enter the data into Lists and Spreadsheet view

2: Hit Menu, Statistics, Stat Calculations, One Variable Statistics…

3. Click OK when number of lists appears. 4. In the pop up, click in the X1 List box and select visits from the drop down list. Hit the Tab key to move to the next box and select freq from the drop down list in the Frequency List box

There is no need to enter data into the other boxes. 5. Click OK.

6. The statistical data appears. The mean is given by �̅� = 1.25 The median is 1. n is useful as it gives the frequency total.


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Grouped Data

When data is presented in a frequency table within class intervals, and we do not know the actual

values within each class interval, we assume that all values are equal to the midpoint of the class

interval in order to find the mean.

Example 5:

The ages of a group of 30 people attending a superannuation seminar are recorded in the frequency

table below, calculate the mean age.

Age (Class Intervals) Frequency

20 - 29 1

30 - 39 6

40 - 49 13

50 - 59 6

60 - 69 3

70 - 79 1

Total 30

To find the mean age, assume all people in the class interval 20 - 29 are 24.5 years of age (This value

is obtained by finding the midpoint of 20 - 29), all people in the class interval 30 - 39 are 34.5 years

of age and so on. The mean age can be found from the table below:

Age (Class Intervals) Frequency 𝒇 Midpoint of Class Interval 𝒎

𝒇 × 𝒎

20 - 29 1 24.5 24.5

30 - 39 6 34.5 207

40 - 49 13 44.5 578.5

50 - 59 6 54.5 327

60 - 69 3 64.5 193.5

70 - 79 1 74.5 74.5

Total: 30 1405

The mean age �̅� = total of f×m

total of f=

1405

30= 46.8 𝑦𝑒𝑎𝑟𝑠 (𝑐𝑜𝑟𝑟𝑒𝑐𝑡 𝑡𝑜 1 𝑑𝑒𝑐𝑖𝑚𝑎𝑙 𝑝𝑙𝑎𝑐𝑒)

The above can be more easily done using Lists and Spreadsheet on the calculator.


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1: In Lists and Spreadsheets view enter the data for the midpoints and the frequency into the first

two columns. Label the columns as shown.

2. Press Menu, Statistics, Stat Calculations, One Variable Statistics.

3. Leave the Number of Lists as 1 and select OK.

4. In the pop up box, click in the X1 List box and select midpoint from the drop down list. Press the

Tab key to move to the Frequency List box and select freq from the drop down list.

5. Press the TAB key to move to the OK button.

The mean �̅� = 46.833 The mean age = 46.8 years


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Measures of Variability or Spread It is useful to be able to measure the spread or variability of the data. How dispersed is the data?

The Range

The simplest measure of spread is the range. The range is the difference between the smallest score

and the largest.

Example 1

The set of data 3, 5, 4, 4, 2, 3, 0, 1, 4, 5 (which gave the number of pets in each of 10 households) has

a range of

5 − 0 = 5

The Interquartile Range (IQR)

The lower quartile, 𝑄1 is 1

4 of the way through the set of data.

The upper quartile, 𝑄3 is 3

4 of the way through the set of data.

The 𝐼𝑄𝑅 = 𝑄3 − 𝑄1


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Example 6

The Standard Deviation

The standard deviation gives a measure of the spread of the data about the mean. The formula to

find the standard deviation is complex and we usually find it directly from the calculator. On the

calculator it is denoted by the symbol 𝑠𝑥 . The bigger the standard deviation, the greater the spread

of data.

Example 7

Find the standard deviation of the set of data: 12, 9, 4, 6, 5, 8, 9, 4, 10, 2. This is the same data as in

Example 6.

1. In Lists and Spreadsheet view, enter the data.

2. Press Menu, Statistics, Stat Calculations, One Variable Statistics. 3. Leave the Number of Lists as 1 and select OK. 4. Enter the data into the pop up box as shown.

5. Click OK.


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The statistical data appears.

The standard deviation is given by 𝑠𝑥 = 3.17805 Notice that the lower quartile is given by 𝑄1𝑋 = 4 and the upper quartile is given by 𝑄3𝑋 = 9 This agrees with the solutions to Example 2.

The 𝐼𝑄𝑅 = 9 – 4 = 5

Stem and Leaf Plots Example 8

The data below shows the weights in kg of 20 possums arranged in order of size:

0.7 0.9 1.1 1.4 1.5 1.6

1.7 1.7 1.8 1.8 1.9 2.0

2.1 2.1 2.2 2.3 2.3 2.5

3.0 3.2


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We can represent this data in a stem and leaf plot as shown below:

Key: 0|7 = 0.7kg

Stem Leaf 0 7 9 1 1 4 5 6 7 7 8 8 9 2 0 1 1 2 3 3 5 3 0 2

In a stem and leaf plot the numbers are arranged in order of size. The key is given as 0|7 kg means

stem 0 and leaf 7 which represents 0.7 kg. You should always include a key in the stem and leaf plot.

When preparing a stem and leaf plot keep the number s in neat vertical columns because a neat plot

will show the distribution of the scores. It is like a sideways bar chart or histogram.

The interquartile range can be found from the stem and leaf plot.

1. Find the median weight. The median weight Q2 is the 2

)120( th score. ie the 10.5th score.

The median lies between the 10th and 11th scores. Count through the data to find the

position of the median. It can be seen from the plot that the median lies between 1.8 and

1.9. The median weight is 2

)9.18.1( = 1.85 kg.

2. The lower quartile Q1 will be the 2

)110( th score in the lower half. ie the 5.5th score in the

lower half. Count through the data to find the position of the lower quartile. Q1 =

2

)6.15.1( = 1.55 kg.

3. The upper quartile Q3 will be the 5.5th score in the upper half of the plot. Count through the

data to find the position of the upper quartile. Q3 = 2

)3.22.2( = 2.25 kg

4. The interquartile range = Q3 – Q1 = 2.25 – 1.55 = 0.7 kg

5. See diagram below:

Key: 0|7 = 0.7kg

Stem Leaf 0 7 9 1 1 4 5 6 7 7 8 8 9 2 0 1 1 2 3 3 5 3 0 2

median Q2 Q1

Q3


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Example 9

Find the interquartile range of the data presented in the following stem and leaf plot.

Key: 15|4 = 154

Stem Leaf 15 4 8 8 16 1 3 3 6 8 17 0 0 1 4 7 9 9 9 18 1 2 3 3 5 7 8 8 9 19 2 7 8 20 0 2

The median is the 2

)130( th score. ie the 15.5th score which lies between 179 and 179. So the

median is 179.

The lower quartile Q1 will be the 2

)115( th score in the lower half. ie the 8th score in the lower half.

Q1 = 168.

The upper quartile Q2 will be the 8th score in the upper half of the data. ie 188.

The interquartile range = Q3 – Q1 = 188 – 168 = 20.

See the diagram above.

Using CAS.

You could check your answers by entering the data into your CAS calculator to determine the

median, lower and upper quartiles.

Boxplots Five-number summary

A five number summary is a list consisting of the lowest score (Xmin), lower quartile (Q1), median

(Q2), upper quartile (Q3) and the greatest score (Xmax) of a set of data.

A five number summary gives information about the spread or variability of a set of data.

Box Plots

A box plot is a graph of the 5-number summary. It is a powerful way of showing the spread of data. A

box plot consists of a central divided box with attached “whiskers”. The box spans the interquartile

range. The median is marked by a vertical line inside the box. The whiskers indicate the range of

scores. Box plots are always drawn to scale and a scale is often attached.

median Q2 Q1

Q3


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Interpreting a Boxplot

A boxplot divides the data into four sections. 25% of the scores lie between the lowest score and the

lower quartile, 25% between the lower quartile and the median, 25% between the median and the

upper quartile and 25% between the upper quartile and the greatest score.

Extreme Values or Outliers Extreme values often make the whiskers appear longer than they should and hence give the

appearance that the data is spread over a much greater range than they really are. If an extreme

value occurs in a set of data it can be denoted by a small cross on the boxplot. The whisker is then

shortened to the next largest or smallest score.

When one observation lies well away from other observations in a set, we call it an outlier.

For example the histogram shows the weights of a group of 5-year old boys. Clearly the weight of 33kg is an outlier.


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Determining whether an Observation is an Outlier To identify possible outliers we can use the following rule:

An outlier is a score, 𝑥, which is either

less than 𝑸𝟏 − 𝟏. 𝟓 × 𝑰𝑸𝑹

or

greater than 𝑸𝟑 + 𝟏. 𝟓 × 𝑰𝑸𝑹

Where 𝑄1 is the lower quartile and 𝑄3 is the upper quartile and IQR is the interquartile range

In summary

An outlier is a score, 𝑥, which lies outside the interval

𝑸𝟏 − 𝟏. 𝟓 × 𝑰𝑸𝑹 ≤ 𝒙 ≤ 𝑸𝟑 + 𝟏. 𝟓 × 𝑰𝑸𝑹

Example 10


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Distribution of Data


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Comparing Two Sets of Data Back to Back Stem and Leaf Plots

Two sets of data can be compared using back to back stem and leaf plots. The data below shows the

life time of a sample of 40 batteries in hours of each of two brands when fitted into a child’s toy.

Some of the toys are fitted with an ordinary battery and some with Brand X. Which battery is best?

Key: 6|9 = 69 hours

Ordinary Brand Leaf

Stem Brand X Leaf

8 6 2 0 0 6 9 9 9 9 8 8 6 4 0 7 3 5 8 8 7 5 3 1 1 1 0 8 2 4 8 9 6 6 4 2 2 2 0 0 9 0 1 4 5 5 9 8 7 5 3 1 1 1 10 0 0 2 5 8 8 9 9 4 2 11 0 0 1 1 3 3 6 7 9 12 1 4 6 6 6 7 8 8 13 3 5 14 6

The spread of each set of data can be seen graphically from the stem and leaf plot. It can be

seen that although brand X showed a little more variability than the ordinary brand the

batteries generally lasted longer.

Parallel Box Plots

The above data can also be compared by using parallel boxplots. The boxplots share a

common scale. Quantitative comparisons can be made between the sets of data.

The 5-Number Summaries of both types of batteries are given below. You can work them

out from the stem plots or by using your calculator.

Brand X

Xmin Lower Quartile Q1 Median Q2 Upper Quartile Q3 Xmax

69 95 109.5 122.5 146

Ordinary Brand

Xmin Lower Quartile Q1 Median Q2 Upper Quartile Q3 Xmax

60 78.5 87.5 97.5 114


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The following parallel boxplots can be drawn to compare the data.

50 60 70 80 90 100 110 120 130 140 150

Brand X

Ordinary Brand

From the box plots it can be seen that:

1. Brand X showed more variability in its performance than the ordinary brand. Brand X range =

77, ordinary brand range = 54. Brand X interquartile range = 27.5 and ordinary brand

interquartile range =19.0

2. The longest lifetime recorded was that of a Brand X battery of 146 hours

3. The shortest lifetime recorded was that of an ordinary battery of 60 hours.

4. Brand X battery median lifetime (109.5 hours) was better than that of an ordinary battery

(87.5 hours)

5. Over one quarter of Brand X batteries were better performers than the best ordinary brand

battery (that is, had longer lifetimes than the longest of the ordinary brand batteries’

lifetimes)

The Mean, Standard Deviation and Normal Distribution The Mean

The mean of a set of data, �̅� , is given by: �̅� =∑ 𝑥

𝑛, 𝑤ℎ𝑒𝑟𝑒, ∑ 𝑥 represents the sum of all the

observations in the data set and n represents the number of observations in the data set.

For example the mean of 4, 7, 9, 12 and 18 is given by:

�̅� = 4 + 7 + 9 + 12 + 18

5= 10

The mean is often used as the everyday average and gives a measure of the centre of a distribution.

The mean is sometimes affected by outliers (extreme values) and the median is often a better

average to use as it is not affected by extreme values.

The Standard Deviation

The standard deviation gives a measure of how the data is spread around the mean. The formula for

the standard deviation is given below:

𝑠 = √∑(𝑥 − �̅�)2

𝑛 − 1

Fortunately, the calculator can be used to find the standard deviation. On the calculator it is denoted

by 𝐬𝐱.

Time in Hours


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The 68%-95%-99.7% Rule applied to the Normal Distribution

Many sets of data are approximately symmetric forming a "bell-shaped" curve. We refer to this type

of data as a normal distribution. Examples include birth weights, people's heights etc. Data that is

normally distributed have their symmetrical bell-shaped distribution centred on the mean value, �̅�.

The above data of the heights of people form a bell-shape and approximates a normal distribution.


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Using the symmetry of the bell-shaped curve and the above rules various percentages can be

calculated.

Z-Scores

To compare scores in different distributions we can make use of z-scores. The z-score, also known as

the standardised score, indicates the position of a score in relation to the mean. It gives the number

of standard deviations that the score is from the mean. A z score can be both positive and negative.

A z-score of 0 indicates that the score is equal to the mean, a negative z-score indicates that the

score is below the mean and a positive z-score indicates that the score is above the mean. A score

that is exactly 1 standard deviation above the mean has a z-score of 1. A score that is exactly 2

standard deviations below the mean has a z-score of −2.

Not all z-scores will be whole numbers; in fact most will not be. A whole number indicates only that

the score is an exact number of standard deviations above or below the mean.

For example, if the mean value of the IQ‘s of a group of students is 100 and the standard deviation is

15, an IQ of 88 would be represented by a z score of −0.8, as shown below.

𝑧 = 𝑥 − �̅�

𝑠=

88 − 100

15= −0.8

The negative value indicates that the IQ of 88 is below the mean but by less than one standard

deviation.


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Using Z-Scores to Compare Data

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Univariate Statistics Summary - learn.stleonards.vic.edu.au · Univariate Statistics Summary Page 1 of 21 Further Maths Univariate Statistics Summary Types of Data Data can be classified