1. A method of measuring similarity of a first object represented by first set of feature values to a second object represented by a second set of feature values, comprising: determining respective feature distance values between substantially all corresponding feature values of the first and second sets of feature values; selecting a subset of the determined feature distance values in which substantially all feature distance values that are selected to be within the subset are smaller in value than feature distance values that are not selected to be within the subset; and summing the feature distance values in the subset to produce a partial feature distance measure between the first and second objects.