assay quality assessment method robust z-prime factor A parameter defining the separation of the noise bands of the negative and positive controls in an experiment using robust statistics. The robust z' is defined as 1-(3*(MAD(pos)-MAD(neg))/abs(median(pos)-median(neg)) where MAD(pos) is the median absolute deviation of the positive control population and MAD(neg) is the median absolute deviation of the negative control population. Typically defined on a per-plate basis but may be defined for an overall assay.