The Z-factor is defined in terms of four parameters: the means () and standard deviations () of samples (s) and controls (c). Given these values (, , and , ), the Z-factor is defined as: 
For assays of agonist/activation type, the control (c) data (, ) in the equation are substituted with the positive control (p) data (, ) which represent maximal activated signal; for assays of antagonist/inhibition type, the control (c) data (, ) in the equation are substituted with the negative control (n) data (, ) which represent minimal signal.
In practice, the Z-factor is estimated from the sample means and sample standard deviations