The Z -factor calculation is useful during piloting for quality assessment of assay conditions (Zhang et. al. 1999). To quantitatively rank assay conditions, perform control experiments and calculate Z’ from the data collected:

Z’ = 1 – 3*(SDpos+SDneg)/abs(Avpos-Avneg);

   SDpos = Standard Deviation of all positive control values
   SDneg = Standard Deviationof all negative control values
   Avpos = Average of all positive control values
   Avneg = Average of all negative control values
   abs represents absolute value.

   1 < Z' < 0.9 an excellent assay
   0.9 < Z' < 0.7 A good assay
   0.7 < Z' < 0.5 Hit selections will benefit significantly from any improvement
   0.5 = Z' The absolute minimum recommended for high throughput screening

This table may differ slightly from published recommendations. However, it is based on the general experience of researchers in this field. We commonly observe that screening results rarely achieve the high-quality levels seen during piloting using controls.

Z’-factor Reference: Ji-Hu Zhang, Thomas D. Y. Chung and Kevin R. Oldenburg (1999). A simple statistical parameter for use in evaluation and validation of high throughput screening assays. J. Biomol. Screen 4:67-73