SSMD (PC x NC)

The SSMD (strictly standardized mean difference) is a statistical parameter that describes the quality of an assay or a screening plate. It takes into account the separation and the variabilities of two controls (e.g. positive and negative control wells) and ranges from plus to minus infinity. The SSMD as an alternative to the Z-Prime factor is a bit easier to interpret because it behaves similar as the Z score.

For groups of data (e.g. wells of plates) SSMDs are calculated for two selected subsets (e.g. positive and negative control wells of a plate). Multiple positive and negative controls can be selected as subsets and the SSMDs will be calculated for all combinations of these. For each group mean and standard deviation (or median and mad) are calculated for the selected subsets.

SSMD = (mean(x[subset{positive}]) - mean(x[subset{negative}])) / sqrt(sd(x[subset{positive}])² + sd(x[subset{negative}])²)
or
SSMD = (median(x[subset{positive}]) - median(x[subset{negative}])) / sqrt(mad(x[subset{positive}])² + mad(x[subset{negative}])²)

Literature: Zhang, X., Genomics 89 (2007) 552–561
Wikipedia: SSMD