The laboratories are evaluated by how closely their analytical results approximate the most probable value of the blind QC samples. Most probable values are the median concentrations reported for each constituent in the round-robin evaluation of the standard reference sample. The assessment of whether an analytical result is acceptable is based on the number of standard deviations that the measured concentration differs from the most probable value. Analytical results that are within two standard deviations of the most probable value are considered acceptable. Bias is systematic error that causes consistently positive or negative deviation in the results from the most probable value.
Variability is random error that affects the ability to reproduce results.
Repeated measurements of the IBSP samples over time provide estimates of both systematic bias and random variability in the laboratory analytical procedures. The IBSP uses a variety of graphical and statistical tools to evaluate laboratory analyses of the blind QC samples. Control charts are produced for each analytical method as a review of the laboratory performance. They are a graphical display of the analytical deviation from the most probable value with respect to time. Control charts show if analytical results are within the expected control limits.