1. A method of calibrating a haptic interface comprising: outputting to a touchable device a signal to cause simulation of a first pattern, determining from user response to said simulation an index number for the first pattern, outputting to said touchable device at least one further signal causing simulation of at least one further pattern and determining from user response thereto a respective index number for each such further pattern to provide a set of user-specific index numbers, and comparing the set of index numbers with a predetermined set of known index numbers representing a base exponent value to determine a mathematical parameter set defining user-specific correction factors to be applied to haptics signaling outputs, wherein the correction factors determined define the Stevens exponent for the user.