Inforbiomatica

I recently reread the original Z factor paper (Zhang et al). The Z factor is a measure of assay reliability and comes in two flavors: the Z’ factor, based entirely based on controls (those with and without the desired effect); and the Z factor, based on experimental data compared with the controls that should have the desired effect.

Rereading a paper months later often makes you wonder whether you read the paper at all the first time. This reading really clarified for me what the Z factor is, that it is not just for high-throughput screening, and raised a number of questions (especially after discussion with colleagues) not addressed in the paper.

The Z factor is the ratio of the “separation band” of the data to the assay dynamic range. A picture helps:

where μ₊ is the mean of the positive controls (in this case the controls with desired effect), μ_s is the mean of the data, σ₊ is the standard deviation of the positive controls, etc. The assay dynamic range in this diagram is μ₊ - μ_s. The screening window is then (μ₊ - μ_s) - (3σ₊ + 3σ_s), and the ratio of this to the dynamic range is the Z factor = 1 - (3σ₊ + 3σ_s)/(μ₊ - μ_s).

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