Quality control

The quality of the analyses is checked by analyzing reference materials and by participating in proficiency testing schemes or intercomparison exercises (QUASIMEME). For each determinant a control chart is kept which compares the analysis result to the true value of the reference material or the robust mean of the intercomparison exercise. For this, the analysis result is given a score (so called z-score) that is representative for the deviation from the “true” value. Analysis result that are equal to the “true” have a score of 0. Analysis results that are higher than that value get a positive score, in the opposite case they get a negative score. An analysis is under control when the z-score is between +2 and -2 (warning limits) and out of control when the z-score exceeds +3 or -3 (control limits). An example of such a chart is shown below.