Abstract: Articles of currency, for example coins, are validated by progressively eliminating candidate target classes in successive classification stages. A Mahalanobis distance associated with a plurality of properties is calculated over successive stages, the results at each stage being used to reduce the number of target classes, and hence the number of calculations required, in the successive stage or stages. Preliminary stages may represent Mahalanobis distance calculations for a sub-set of the measurements represented by the final Mahalanobis distance calculation. Thus, the Mahalanobis distance calculation can be started before some of the measurement parameters required for the later stages are available.

Abstract: Articles of currency, for example coins, are validated by calculating a Mahalanobis distance associated with a plurality of properties in successive stages, the results at each stage being used to reduce a number of target classes, and hence the number of calculations required, in the successive stage or stages. Preliminary stages may represent Mahalanobis distance calculations for a sub-set of the measurements represented by the final Mahalanobis distance calculation. Thus, the Mahalanobis distance calculation can be started before some of the measurement parameters required for the later stages are available.