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For measures of similarity between samples based on species presence-absence, the observations can be summarised in a simple frequency table:
where the number of species present in both samples is a, the number of species present in sample 1 but missing from sample 2 is b, the number of species missing in sample 1 but present in sample 2 is c and the number of species missing from both samples is d. The total number of species, N, is therefore a+b+c+d.
Simple matching similarity is (a+d) / N.
Note that this measure gives equal weighting to double zeros and species which are present in both samples. This is rarely useful in ecological studies.