1. A computer system operable to determine variance data for a plurality of different locations in a reference medical image, the variance data representing a statistical variation among corresponding locations identified in a plurality of training medical images, the computer system comprising: a memory adapted to store data representing the reference medical image and the plurality of training medical images; and processing circuitry operable to execute machine readable instructions to: receive an indication of locations of a plurality of anatomical features in the reference medical image and an indication of locations of corresponding anatomical features in respective ones of the training medical images; determine offsets between respective ones of the locations of the anatomical features in the reference medical image and locations for the corresponding anatomical features in respective ones of the training medical images; and determine variance data for respective ones of the locations of the anatomical features in the reference medical image from a statistical analysis of the determined offsets.