The Z-factor defines a characteristic parameter of the capability of hit identification for each given assay. The following categorization of HTS assay quality by the value of the Z-Factor is a modification of Table 1 shown in Zhang et al. (1999); note that the Z-factor cannot exceed one.
Note that by the standards of many types of experiments, a zero Z-factor would suggest a large effect size, rather than a borderline useless result as suggested above.  For example, if σ=σ=1, then μ=6 and μ=0 gives a zero Z-factor.  But for normally-distributed data with these parameters, the probability that the positive control value would be less than the negative control value is less than 1 in 10.  Extreme conservatism is used in high throughput screening due to the large number of tests performed.