Decision threshold and detection limit: estimation, interpretation and optimization. 1. part: basic principles

The use of decision threshold and detection limit concepts often poses many problems for metrologists in biological analysis laboratories in charge of the management of nuclear plant workers. These problems are frequently related to the understanding of a normalized formula or to the possible choice between several formulas whose numerical results are exceptionally significantly different. The true problems are in fact firstly the significance and the interpretation of these statistical tests, and secondly the numerical values used in these tests. Among those, the background estimate remains the essential and the most delicate value to define. The aim of this first article is to go back to the initial bases of the concepts of the decision threshold and detection limit. In the first step, a distinction will be made between the methods of decision threshold determination based upon definition of a priori first species error risks (i.e. before sample measurement) and the a posteriori interpretation of the sample measurement result. In the second step, some examples of decision threshold and detection limit optimization in gamma spectrometry will be described. These examples will show that the essential parameter is the optimization of the estimation of background values, which requires the control of measurement rather than the management of statistical tools. (authors)