Groups

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9.3

Similarity Percentages (SIMPER)

To undertake this test you must first have defined the group membership of the

individual samples (see 

Edit Groups

45

).

This analysis breaks down the contribution of each species to the observed

similarity (or dissimilarity) between samples. It will allow you to identify the

species that are most important in creating the observed pattern of similarity. The

method uses the 

Bray Curtis

94

 measure of similarity, comparing in turn, each

sample in Group 1 with each sample in Group 2. The Bray Curtis method

operates at the species level and therefore the mean similarity between Groups 1

& 2 can be obtained for each species.

In the following example, using the Powerstation_fish.csv data file the 18 power

stations (samples) have been divided into four arbitrary groups.

The SIMPER results for Group 1 are shown below (SIMPER within tab). Note that

these are WITHIN groups similarities and shows that although the average

abundance of Flounder is much less than for Sprat, in terms of similarity between

all the samples in the group it is slightly higher. In view of the very large disparity

in average abundances, it may be worth experimenting with various data

transformations such as square root.

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