Measurement data is obtained for calibration fields that have been exposed by a lithographic apparatus using different field layouts and exposure sequences. The measurement data is classified in subsets by scan direction, step direction, field size and other variables. The measurement data is indexed by a time value that varies through each exposure sequence. Time values within different exposure sequences can be related using a normalized time value based on the beginning and end of each exposure sequence. An inter-field performance model is calculated for each subset. An intra-field component of a performance model is calculated with time as a third dimension. The time-indexed performance model is used to determine intra-field corrections for a variety of product exposures having product layouts and product exposure sequences different to the calibration fields, based on time and other a variables of the product layout and product exposure sequence.