The Z-score or standard score is the signed number of standard deviations (SD) by which the value of an observation or data point is above the mean value of what is being observed or measured. See Wikipedia for details. We implement a robust version of the Z-score where the SD is replaced by MAD and mean by median.

A desirable compound should exhibit large values of efficacy and smaller values for potency. Thus, in this plot, the efficacy curve is ideally located at higher values of Z and the potency curve at smaller (i.e., more negative) values of Z.