PiscesLogoSmallerStill  Russell & Rao

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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:

 



Sample 1




Species Present

Species Absent

Sample 2

Species Present

a

b


Species  Absent

c

d

 

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.

 

This measure is calculated as: a/N