1. A method of deriving a classifying function for classifying items of currency, the method comprising deriving a plurality of measurements from at least one currency sensor and a plurality of currency items, and forming a set of data from the measurements, the elements of the data set corresponding to points in a first space, and determining a set of values corresponding to the measured values, selecting a kernel function corresponding to a mapping of the first space to a second space, the method comprising deriving an expression representing the function of the measured values in terms of the kernel function and using it to derive a classifying function.