1. Apparatus for associating gaze data representative of the location of gaze of an operator with classifications of images of an operator looking at different locations, comprising: a receiver operable to receive a video stream defining a sequence of images representative of an operator at different points in time; a classification unit operable to assign one of a number of classifications to images in the video stream received by said receiver, said classification unit being operable to assign the same classifications to images of an operator looking at the same locations; and a calibration unit comprising: a classification store configured to store data identifying the pairs of different classifications assigned to pairs of images in a video stream received by said receiver representative of an operator at different times separated by less than a preset time period; a gaze conversion store configured to store data associating each of said number of classifications with gaze data representative of a gaze location; and an update unit operable to update gaze data stored in said gaze conversion store by updating said gaze data such that gaze data for pairs of different classifications identified by data stored in said classification store identify gaze locations which are closer together and gaze data for pairs of different classifications not identified by data stored in said classification store identify gaze locations which are further apart.