The Discriminatory Power (D) is the average probability that the typing system
will assign a different type to two unrelated strains randomly sampled in
the microbial population of a given taxon.
This tool has been created to help calculate this parameter 
The Discriminatory Power (D), as shown by Hunter can be expressed by the
formula of Simpsonīs index of diversity, which reads:
Where D is the index of discriminatory power, N the number of unrelated strains tested, S the number of different types, and xj the number of strains belonging to the jth type, assuming that strains will be classified into mutually exclusive categories. Thus, a D value of 1.0 would indicate that a typing method was able to distinguish each member of a strain population from all other members of that population. Conversely, an index of 0.0 would indicate that all members of a strain population were of an identical type. An index of 0.50 would mean that if one strain was chosen at random from a strain population, then there would be a 50% probability that the next strain chosen at random would be indistinguishable from the first.
References:
Hunter PR and Gaston MA. Numerical index of the discriminatory ability of typing systems: an application of Simpsonīs index of diversity. J Clin Microbiol. 1988 November; 26(11): 24652466. PubMed
