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, and source code is available at biophp.org 

Example: a novel typing system was used to type 100 randomly sampled E. coli strains.
Four types were obtained, and to each type the following number of strains was assigned:
Type A: 40 strains
With this data, discriminatory power was calculated and its value was 0.7071 