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This window allows you to compare the variables present in two samples, or the group membership of two species. It is particularly suitable for comparing the species present in different samples, but can also be used to compare types of pottery, fossils etc. When used to compare group membership, it can be used to help determine which species characterise a particular group.

 

Comparing two samples is a straightforward operation; each sample will contain a subset of the full list of species present in all the samples, and in most cases, the two subsets of species will overlap; it is a Venn diagram in list form. There is a fourth category which is not shown by this function - species which do not occur in either of the two samples being compared.

 

comparing samples2

 

In the case of the Groups comparison, it is the variables' membership of groups, rather than the groups themselves, which is being compared. In the following example, using the River inverts demo data set, we are comparing the occurrence of the Agriidae (the demoiselles) with the Astacidae (freshwater crayfish); the Agriidae occur in  a Group 2 sample, Agriidae and Astacidae occur together in one of the samples of Groups 3 and 4, and Astacidae occur without the Agriidae in a sample from Group 1.

 

To continue our Venn diagram analogy, this function of CAP categorises a group in one of 3 ways: "Agriidae only", "Agriidae+Astacidae", and "Astacidae only":

 

comparing samples3

 

As in the previous example, there remains a fourth classification, which is not shown: "Not-Agriidae+Not-Atacidae", groups where neither variable occurs.