The degrees of equivalence are the main outcome in the analysis of key comparison data, and they are used for the approval of the calibration and measurement capabilities of the participating laboratories. Typically, the calibration and measurement capability of a participating laboratory is seen as being approved when the corresponding unilateral degree of equivalence does not differ significantly from zero.
The relevance of degrees of equivalence may deteriorate in the presence of an instability of the common measurand. In order to quantitatively assess this deterioration we propose to consider the loss of power of a hypothesis test that can be associated with checking whether a degree of equivalence differs significantly from zero. Based on the resulting loss of power, one can decide whether the size of the instability of the common measurand may be tolerated. We illustrate the concept in terms of results obtained for the recent key comparison CCM.FF-K6.2011.