Sampling:The Hows and Whys

Sampling:The Hows and Whys

Driven to Discover

  Enabling Student Inquiry through Citizen Science

Driven to Discover

  Enabling Student Inquiry through Citizen Science

SamplingSampling

Sampling: to collect data on a subset of a population

From samples we make inferences about the overall population.

Sample size: as the fraction of the total population included in the sample rises, the greater the confidence the researcher can have in the accuracy of the inferences.

Why sample?Why sample?

Unless you plan on measuring every individual you will need to select a sub-population for investigation

Unless you plan on measuring every individual you will need to select a sub-population for investigation

Why sample?Why sample?

Unless you plan on measuring every individual you will need to select a sub-population for investigation

Field of milkweed: how tall are they Measure them all?

Unless you plan on measuring every individual you will need to select a sub-population for investigation

Field of milkweed: how tall are they Measure them all?

A complete count may be possible here….

But not here. Sampling may be needed. This situation is more common in science.

Why sample?Why sample?

Unless you plan on measuring every individual you will need to select a sub-population for investigation

BUT HOW???

Random or structured?Random or structured?

Samples can be selected in either a random or a structured manner. Some studies and sites lend themselves to one

or the other Deciding random vs. structured is one of your

first major decisions.

Samples can be selected in either a random or a structured manner. Some studies and sites lend themselves to one

or the other Deciding random vs. structured is one of your

first major decisions.

StructuredStructured

Every n’th bush, bird, etcImportant that the starting point be randomly

chosen, however.

This is not random, but structured. The “nth” determined by fraction of total

population you want included.

Every n’th bush, bird, etcImportant that the starting point be randomly

chosen, however.

This is not random, but structured. The “nth” determined by fraction of total

population you want included.

StructuredStructured

Plots at a set distance and compass heading eg. every 20 meters in a N-S/E-W grid The starting point can be

logically chosen: the gate we walk though, or 20 feet in from the corner of the site

Randomly chosen

Plots at a set distance and compass heading eg. every 20 meters in a N-S/E-W grid The starting point can be

logically chosen: the gate we walk though, or 20 feet in from the corner of the site

Randomly chosen

RandomRandom

All bushes, birds, etc have an equal chance of selectionDraw numbers from a hat….

Plots at a random distance and compass headingEach iteration will be unique

All bushes, birds, etc have an equal chance of selectionDraw numbers from a hat….

Plots at a random distance and compass headingEach iteration will be unique

HybridHybrid

A combination of structured and random selection. This is fairly common. Plots could be laid out along a transect at set

distances, but the direction of the transect could be randomly determined.

A combination of structured and random selection. This is fairly common. Plots could be laid out along a transect at set

distances, but the direction of the transect could be randomly determined.

NEVER! arbitraryNEVER! arbitrary

Structured or random selection are OK, but it is NEVER permissible to choose arbitrarily Arbitrary: one here, one over there, one by the corner…. Structured

Defined rules for selection Random: no control or decisions by the researcher.

Usually with no rules* It is legitimate to require that no 2 random sites be within a set

distance of each other.

Easy to confuse random and arbitrary, but they are not the same.

Structured or random selection are OK, but it is NEVER permissible to choose arbitrarily Arbitrary: one here, one over there, one by the corner…. Structured

Defined rules for selection Random: no control or decisions by the researcher.

Usually with no rules* It is legitimate to require that no 2 random sites be within a set

distance of each other.

Easy to confuse random and arbitrary, but they are not the same.

Rules for bothRules for both

Regardless of whether you are using a structured or random assignment, what you get is what you get!

Do NOT! skip a plot because it looks too hard, or does not have any plants, and do not choose points because they look “good”

Regardless of whether you are using a structured or random assignment, what you get is what you get!

Do NOT! skip a plot because it looks too hard, or does not have any plants, and do not choose points because they look “good”

When to use structured assignment

When to use structured assignment

Structured assignment of samples is good sampling, and appropriate in some cases.

Often used in bird surveys

Also good if doing plots inside a single, larger study site, especially if the plots will be permanent.

Structured assignment of samples is good sampling, and appropriate in some cases.

Often used in bird surveys

Also good if doing plots inside a single, larger study site, especially if the plots will be permanent.

When to use random assignment

When to use random assignment

Random sampling is the gold standard

Best if wanting to make inferences from a population of discrete, obvious individuals

Also used in identifying research sites

Random sampling is the gold standard

Best if wanting to make inferences from a population of discrete, obvious individuals

Also used in identifying research sites

Stratified SamplingStratified Sampling

Choosing random samples from 2 or more classifications inside the overall population

Have equal numbers of men and women Select research sites of different classes

proportional to the ratio of that class

Choosing random samples from 2 or more classifications inside the overall population

Have equal numbers of men and women Select research sites of different classes

proportional to the ratio of that class

How to achieve randomnessHow to achieve randomness

How you achieve randomness will depend on project, here we assume random allocation of plots Computerized maps to randomly choose plots Hand drawn and manually selected plots Random distance and direction

How you achieve randomness will depend on project, here we assume random allocation of plots Computerized maps to randomly choose plots Hand drawn and manually selected plots Random distance and direction

ComputersComputers

GIS software--such as Arc-Map will allow you to map your sites, define plots, or choose random points

GIS software--such as Arc-Map will allow you to map your sites, define plots, or choose random points

Hand drawn plotsHand drawn plots

Map your study site on paper Break it into blocks

Columns, rows Randomly select row and column (draw

numbers from a hat, or other random selection)

Number every potential plot, randomly draw Repeat until the needed number of plots is

selected.

Map your study site on paper Break it into blocks

Columns, rows Randomly select row and column (draw

numbers from a hat, or other random selection)

Number every potential plot, randomly draw Repeat until the needed number of plots is

selected.

Random distance and directionRandom distance and direction

From a starting point (the center of the site, an edge, the

trailhead, or a random point itself) generate a random compass heading. Spin a wheel, draw numbers 1-12 (the clock) from a

hat, spin a stick or pencil…. Proceed in the direction indicated

Repeat as needed. Distance can be standardized (or more rarely

also random

From a starting point (the center of the site, an edge, the

trailhead, or a random point itself) generate a random compass heading. Spin a wheel, draw numbers 1-12 (the clock) from a

hat, spin a stick or pencil…. Proceed in the direction indicated

Repeat as needed. Distance can be standardized (or more rarely

also random

Random concernsRandom concerns

Before starting you will need to define rules for the plots How close can they be together Is stratification necessary How many will you need

These need to be defined before starting selection and should have a logical, scientific rationale.

Before starting you will need to define rules for the plots How close can they be together Is stratification necessary How many will you need

These need to be defined before starting selection and should have a logical, scientific rationale.

Sample SizeSample Size

As we discussed, larger samples relative to the population will lead to more confident predictions But also more work

The key is to find a sample size large enough to give you statistical significance. Ask the Monarch Lab or your science mentor

for help in determining adequate sample size.

As we discussed, larger samples relative to the population will lead to more confident predictions But also more work

The key is to find a sample size large enough to give you statistical significance. Ask the Monarch Lab or your science mentor

for help in determining adequate sample size.

Have Fun!Have Fun! Do good science!

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