percentage/potency efficiency index (pei), binding efficiency index (bei) and surface-binding efficiency index (sei) because they are easier to calculate and take into account the differences between elements in different rows of the periodic table. it is important to note that pei is a relative measure for comparing compounds tested in the same conditions (e.g. a single-point assay) and are not comparable at different inhibitor concentrations. also for bei and sei, similar measurements must be used (e.g. always using pk).