A method of testing currency e.g. coins comprises taking at least two measurements of an unknown coin: and accepting said unknown coin as corresponding to a first coin type A if the point defined by the measurements lies, in the space defined by the axes of the measurements, on a first side of a discriminant surface C at which the Mahalanobis distances of the centres of the distribution of said first coin type A and a second coin type B are in a predetermined relationship, characterised in that said relationship corresponds (at least approximately) to equality of the sums of the respective Mahalanobis distances with respective predetermined constants at least one of which is non-zero. In this way the relative probabilities of the two coin types occurring in a population can be taken into account. In other described embodiments the discriminant surface is changed depending on the characteristics of articles previously measured. Alternatively an acceptance region is adjusted in accordance with the measurements of articles previously tested or in dependence on the time between successive valid and/or invalid articles, or in dependence on the probability that an article just tested is valid or not.