Similarity Percentages (SIMPER)
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To undertake this test, you must first have defined the group membership of the individual samples (see Allocating samples to groups).
This analysis breaks down the contribution of each species (or other variable) 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 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 Romano British pottery.csv data file, the data have been divided into 4 location groups.
The SIMPER results for the SIMPER Within tab shown below indicate that Al (aluminum) is the variable that contributes the most to the within-group similarities at every site.
Below, the results for the between-groups analysis is shown (SIMPER Between tab). There is one of these results panels for each pair-wise combination of groups. Note that on this panel it is actually the dissimilarity that is displayed.