Assessment of "Average of Normals" quality control procedures and guidelines for implementation.

The capabilities of "Average of Normals" control procedures have been assessed by determining power functions, graphs of the probability of error detection versus the size of analytic error. The power functions indicate that the most important determinants of statistical power are the ratio of the standard deviation of the patient population (Sp) to the analytic standard deviation (Sa), Sp/Sa; the number of data points averaged (N); the control limits (probability for false rejection); truncation limits for selecting the population; and the magnitude of the population lying outside the truncation limits. General guidelines for the implementation of "Average of Normals" are provided, along with a nomogram for the selection of N as a function of Sp/Sa and the probability of error detection. Optimal performance of these procedures may require simulation studies on a per analyte basis.