Quality control in clinical chemistry: the two-sample plot and improvement of laboratory performance.

A two-sample plot is often used to display test results in clinical chemistry external quality control programs. The use of control materials at different concentrations calls for a re-evaluation of the two-sample plot. A modification of the two-sample plot is presented. The axes of the graph are graduated in units of measurement rather than standard deviation intervals. The mean of all laboratories or a reference laboratory's value is entered as the target value. For each laboratory, repeated results for the same or similar control materials are recorded. Visual inspection of the graph gives immediate information as to the extent of intra-laboratory variability, the incidence of systematic errors in the laboratory, and the size and type of systematic errors (proportional or constant) with respect to the target value.