Why is iis using target z-score and not the usual z-score calculations in which standard deviation of the group of laboratories is used?
This way of z-score calculation results in a simple, straight forward comparison of lab results with the reproducibility stated in the corresponding international accepted test method. It indicates how many times the standard deviation the reported result deviates from the ‘true value’. Target z-scores allow evaluation of performance over longer periods using a series of pt-results. This is impossible with z-score calculated from the standard deviation of the group of participants as the group of participants is not constant, nor is its performance. See also R.G.Visser, W.Oussoren, Accred Qual Assur 3:497-498.
