1. A computer implemented method of comparing a first representation of an identifier with a second representation of an identifier, wherein the identifier is a biometric identifier or a form of identifier specific to a person or item, and in which the first representation and second representation is a fingerprint, palm print, ear print, retina image, footprint, shoe print, tool mark, or a part of any of these, the method comprising: converting the first representation to an expression of the first representation, wherein the expression of the first representation is a sub-set of the data contained within the first representation; generating one or more variant expressions from the expression of the first representation, wherein the one or more variant expressions are generated from the expression of the first representation to simulate the effect of a source of variation upon the expression of the first representation; converting the second representation to an expression of the second representation, wherein the expression of the second representation is a sub-set of the data contained within the second representation; providing a plurality of other representations and converting each of the other representations to an expression of the other representations, wherein the expressions of the other representations are a sub-set of the data contained within those other representations; generating one or more variant expressions from each expression of the plurality of other expressions, in which the expressions of the other representations are a sub-set of the data contained within those other representations; defining a boundary with respect to the expression of the second representation, the boundary defining a space inside the boundary; determining a first value, the first value being an expression of the number of the variant expressions of the expression of the first representation within the space defined by the boundary; determining a second value, the second value being an expression of the number of variant expressions of the expressions of the other representations within the space defined by the boundary; determining a measure of comparison between the first representation of the identifier and the second representation of the identifier from the first value and the second value, in which the measure of comparison is the ratio of the first value to the second value.