1. A method for determining whether a predetermined sign was made by a specified person, the method comprising the steps of obtaining a succession of position signals representing the position of a writing instrument used in making the sign, filtering the position signals using a filter having a lower cut-off frequency greater than 0.25 Hz and an upper cut-off frequency less than 20 Hz, segmenting the filtered signal on the basis of a characteristic of the filtered signal, deriving at least one group of feature signals in which each signal is associated with a respective segment of the filtered signal and represents one characteristic of the segment, each other signal in the group representing the said one characteristic for the associated segment, storing a group of reference signals, each associated with the same respective segments as the feature signals and derived from a number of operations of writing the predetermined sign by the specified person, correlating the feature signals and reference signals by comparing respective signals in each of a plurality of sub groups of feature signals and reference signals associated with the same segment to obtain an overall error for the sub group, and systematically changing the feature signals associated with respective segments without changing the sequence of feature signals until the overall error is within a predetermined limit or the least possible overall error is obtained, and comparing respective feature signals and reference signals associated with the same segment after correlation to obtain a further overall signal for at least most of the feature signals and providing an indication of whether a sign was apparently made by the specified person on the basis of the further overall error.