Abstract. The IRA and the NPL have submitted ampoules of ¹⁶⁶Ho^m to the International Reference System (SIR) for activity comparison at the Bureau International des Poids et Mesures, thus becoming the third and fourth participants since 1989. The five samples of known activity of ¹⁶⁶Ho^m now recorded in the SIR have activities between about 70 kBq and 500 kBq. The new results have enabled a re-evaluation of the key comparison reference value, and the degrees of equivalence between each equivalent activity measured in the SIR and the key comparison reference value (KCRV) have been calculated. The results are given in the form of a matrix for these four NMIs together with the recalculated degrees of equivalence of an APMP regional comparison held in 2000, comparison identifier APMP.RI(II)-K2.Ho-166m for six other NMIs. A graphical presentation is also given.

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The final report has been peer-reviewed and approved for publication by the CCRI Section II, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).