Disclosed is a method of determining a correction for a measurement of a target and an associated apparatus. The measurement is subject to a target-dependent correction parameter which has a dependence the target and/or a stack on which the target is comprised. The method comprises obtaining first measurement data relating to a measurement of a fiducial target, said first measurement data comprising at least a first and second set of intensity parameter values: and second measurement data relating to a measurement of the fiducial target, the second measurement data comprising a third set of intensity parameter values. A target-invariant correction parameter is determined from said first measurement data and second measurement data. the target-invariant correction parameter being a component of the target-dependent correction parameter which is not dependent on the target and/or a stack: and the correction is determined from said target-in-variant correction parameter.