# STATISTIClAL METHODS A2 BIOLOGY

STATISTIClAL METHODS A2 BIOLOGY

USEFUL FOR PRACTICALS|! STATS M,ETHOD

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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?
(eg. predict the pollutant levels at
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Which Stats test should I use? Bio Factsheet
Table 1: Statistical tests for various investigations
INVESTIGATION WHAT IS MEASURED ? NULL HYPOTHESIS STATISTICAL EXPLANATION
TEST
1. Effect of pH on Number of seeds (out of 20, say) Ho: Number of seeds Chi-squared We are comparing observed
seed germination germinating in each of several germinating is not frequencies with expected
trays which contain different pH dependent on pH frequencies (i.e. that the
solutions same number germinate in
each tray)