For each species i in group j, define the specificity as
Aij = Nij / Ni
where Nij is the mean number of individuals of species i across sites in group j, and Ni is the sum of the mean numbers of individuals of species i over all groups.
Similarly, define the fidelity as
Bij = Nsitesij / Nsitesj
where Nsitesij is the number of sites in group j where species i is present, and Nsitesj is the total number of sites in group j.
The indicator value of species i in group j is then a value from 0 to 100 (percentage):
INDVALij = 100 Aij Bij.
The statistical significances (p values) of the indicator values are estimated by 9999 random reassignments (permutations) of sites across groups. The p values can optionally be Bonferroni corrected (multiplied with the total number of indicator values, this is highly conservative).