The ?show=formula">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 
$val){ $temp+=$val*($val1); $N+=$val; } $result["D"]=round(1($temp/($N*($N1))),4); $result["N"]=$N; $result["types"]=sizeof($data_array); return $result; } //######## is integral function str_is_int($str) { $var=intval($str); return ("$str"=="$var"); } //########print form function print_form($values,$N,$S,$result){ ?>

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 
Reference
Hunter P. Reproducibility and indices of discriminatory power of microbial typing methods. J Clin Microbiol 1990; 28: 19035. PubMed. A Discriminatory Power (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. This statistical was originally described by Simpson (1954).
