“Isopoda” by Wikipedia under CC by 2.0


  1. To recognize the diversity of experimental designs that can be used to test a single hypothesis
  2. To understand the difference between descriptive and inferential statistics
  3. To understand the difference between common statistical tests
  4. To practice designing experiments with consideration of the specific inferential statistic used before collecting the data
  5. To practice critically evaluating experimental designs


Materials & Methods

  • 5 groups of 5 isopods
  • Stopwatch
  • Dry potting soil
  • Wet potting soil


Hypothesis: Isopods prefer moisture

Prediction: If isopods prefer moisture, then more individuals will move towards the wet side of the substrate than to the dry side.

Null Hypothesis: Isopods do not have a preference between wet and dry soil.


Observed Frequency Table

Trial Number Isopods In Wet Isopods In Dry Not In Either
1 2 3 0
2 2 1 2
3 4 0 1
4 2 3 0
5 5 0 0


Average In Wet Average In Dry Average In Neither
3 1.4 0.6

Based on the data that was collected, more isopods were seen in the wet soil than the dry soil. We used statistical testing to prove/disprove our prediction. The Mann-Whitney U test was used. We were able to conduct this test because we had two sets of unpaired data.


To start this test, each value was ranked. This is shown in the chart below.

Mann-Whitney U Test

Dry Soil Rank Wet Soil Rank
0 1.5 2 5
0 1.5 2 5
1 3 2 5
3 7.5 4 9
3 7.5 5 10


Then the sum of the ranks for each group was taken

-Nwet = ∑ 5+ 5 + 9 + 10 =34

-Ndry = ∑1.5 + 1.5 + 3 + 7.5 + 7.5 =21

The larger number (34) is used to calculate the test statistic. The calculated test statistic was 19, and the critical value for a data set with 5 and 5 value is 21.

If there was significance, the critical value is less then the test statistic. This is not what we saw. The results showed that the test statistic was 19 and the critical statistic was 21.

This means that we accept our null hypothesis that isopods do not have a preference between wet and dry soil.

We do in fact know that isopods prefer wet soil over dry soil. We could construct a different experiment with different variable and use a different statistical test to confirm that they prefer wet soil.



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