Groups
49
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.
Copyright 2004, PISCES CONSERVATION LTD





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