To obtain a more representative value for the repeatability precision perform the simultaneous replicates at different times (but the same day), on different matrices, at different concentrations. Calculate the standard deviation of repeatability from at least 5 pairs of values obtained from at least one pair of replicates analyzed with each batch of analyses for each pertinent concentration level that differs by approximately an order of magnitude and conducted at different times. The object is to obtain representative values, not the “best value”, for how closely replicates will check each other in routine performance of the method. Therefore these sets of replicate analyses should be conducted at least in separate runs and preferably on different days. The repeatability standard deviation varies with concentration, C expressed as a mass fraction. Acceptable values approximate the values in the following Table or calculated by the formula: