The conditions under which results from different key comparisons in gas analysis can be combined are defined. The paper outlines the underlying model for computing degrees of equivalence. The key point in the development of the relevant expressions is the use of the error propagation law as given in the Guide to the Expression of Uncertainty in Measurement 1993 1st edn (Geneva: ISO), to convert the error model into an uncertainty model, including covariances. The covariance term in the generic expression derived is the key to developing tailored expressions for different cases. Some typical instances are explored, including the linking with a bilateral key comparison and the linking with an RMO key comparison. Additionally, a basis is established for linking two comparisons with different pilot laboratories.
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