# STATISTIClAL METHODS A2 BIOLOGY

STATISTIClAL METHODS A2 BIOLOGY

USEFUL FOR PRACTICALS|! STATS M,ETHOD

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- Created on: 15-10-12 16:34

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B io Factsheet

September 1997 Number 3

Which Stats test should I use?

"Which statistical test should we use?" is a common question from Biology students. This Factsheet provides simple guidelines on when

each type of statistical test should be used.

The choice of the correct statistical test is all-important - use the wrong test and the conclusions will be invalidated. Marks are only awarded for an appropriate

- i.e. correct - use of statistics. The flowchart below can be used to identify the appropriate test. Table 1 overleaf gives examples of investigations. and

appropriate tests.

Figure 1. Deciding which test to use

Whether the frequencies are

the same in each of two or

more categories? Chi-squared

(eg. are seed germination rates Goodness-of-Fit

the same in different pHs?)

Comparing frequencies

(numbers of things) in

various categories? Do you Whether observed

(eg. are the numbers of want to frequencies are the same as

seeds germinating in test those theoretically expected?

various trays significantly (eg. are the predictions of

different?)

genetics correct?)

Whether two factors are Chi-squared

related? Contingency

(eg. does pollution affect the table

number of sites at which clinging

Are mayfly are found?)

èyou Calculated

Mann-Whitney

(such as a diversity index) or

Counted U test

(such as the number of

organisms?)

Finding if there is a Yes paired

difference between two t-test

Is the

averages? Do the data occur in

data

(eg. is there, on average, a Measured natural pairs?

higher species diversity in (such as length, width,

unpolluted water rather height, velocity)? eg. the same organism

than polluted water?) reacting to two different

stimuli.

No unpaired

t-test

Find whether the two

variables are correlated? Spearman's

(i.e does increasing one cause rank correlation

the other to increase or coefficient

Investigating the decrease?)

relationship between

two variables? Do you

(eg. Is there a relationship want to Use one variable to predict

between pollution level the value of the other?

and distance from road?) Regression

(eg. predict the pollutant levels at

10m from the road)

1

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