Dimensionless statistical parameter that provides a practical assessment of assay performance in the presence of test compounds. \[Z = 1 - 3(\sigma_{\rm{s}} + \sigma_{\rm{c}})/\vert\mu_{\rm{s}} - \mu_{\rm{c}}\vert\] where \(\mu_{\rm{s}}\) denotes the mean of the library sample signal, \(\mu_{\rm{c}}\) the mean of the control signal and \(\sigma_{\rm{s}}\), \(\sigma_{\rm{c}}\) the corresponding standard deviations. \(1 \gt Z \ge 0.5\) generally categorizes an excellent assay.