Abstract
Guidelines containing two procedures are proposed for the evaluation of key comparison data. They apply to the simple circulation of a single travelling standard around all the participants. The application of the procedures to a specific set of key comparison data provides a key comparison reference value (KCRV) and the associated uncertainty, the degree of equivalence of the measurement made by each participating national institute and the degrees of equivalence between measurements made by all pairs of participating institutes. Procedure A is based on the use of the weighted mean, together with consistency checks based on classical statistics regarding its applicability. Should the checks fail, action to remedy the situation is suggested. If the remedy is inappropriate, Procedure B can be applied instead. It is based on the use of the median (or any another informed choice) as a more robust estimator in the circumstances.