A reference data set used for classifying items of currency is established by obtaining at least one measured response from at least one known item of currency, projecting the measured response data from the known item of currency from a first space to a second space, and applying a reduction technique to the second space thereby reducing the complexity of the second space. The reference data set, which can be used for classifying items of currency, is thus represented by a subset of all measured responses obtained from the known item of currency.