Evaluation of results derived from the analysis of certified reference materials - a user-friendly approach based on simplicity

Certified reference materials (CRMs) have now been in regular use fur sever al decades. Their production and certification are regulated by internation al standards. But, even today there are no agreements on procedures for eva luating results obtained by the users. As a consequence, the way CRM result s are treated in the literature leaves a lot to be desired. A statistical e valuation is rarely, if ever, described in published reports. The most comm on approach is to compare the found mean and/or range with the certified ra nge and then state if the mean falls within the certified range, or if the two ranges overlap. If this happens, the analyst is usually satisfied. In a ddition, usually no regard is paid to the fact that the certified interval is based on a 95% confidence interval (CI) and the found interval on standa rd deviation and that this evaluation has little, if any, statistical relev ance. Long-term evaluation of a CRM often consists in nothing more than pro ducing a control chart, which relates the found results to the certified me an and CI. This paper is an attempt to improve the situation by providing a set of eas y-to-use guidelines for evaluating results from CRMs. During the process we have identified different areas in which there is a need for such guidelin es: 1. short-term evaluation of a single, or multiple, determination at one or several specific times; 2. identification of systematic and random errors; 3. evaluation of CRMs when used in a collaborative trial of a method; and 4. long-term evaluation for monitoring an analytical process over extended periods of time. It is important that the guidelines do not require expert competence in sta tistics from the analyst. Such obstacles would probably render most guideli nes unused.