The MSR is a statistical parameter that characterizes the reproducibility of potency estimates from in vitro concentration-response assays. Biological activity expressed as potency of compounds is an important parameter in screening and drug discovery. Standard potency estimates are expressed as IC50, EC50 or simply as AC50 values derived from concentration-response (CRC) assays designed to measure activation, inhibition or modulation of targets and pathways of pharmacological significance. Concepts of Minimum Significant Ratio (MSR) discussed below address how the reproducibility of potency values is derived quantitatively using standard or control compounds of known activity and test compounds that have been tested in multiple runs. MSR values inform the scientists the extent of reliability of assays designed to support new compound selection through Structure-Activity Relationships (SAR) of chemical libraries.