1. A method of measuring a parameter of a lithographic process, the method comprising: illuminating target structures on a substrate, the target structures comprising at least a first target structure comprising an overlaid periodic structure having a first deliberate overlay bias and a second target structure comprising an overlaid periodic structure having a second deliberate overlay bias; detecting radiation scattered by each target structure to obtain for each target structure an asymmetry measurement representing an overall asymmetry that includes contributions due to (i) the deliberate overlay bias in the target structure, (ii) an overlay error in a lithographic process during forming of the target structure and (iii) feature asymmetry within one or more of the periodic structures; repeating the illuminating and detecting for a plurality of different illumination conditions to obtain asymmetry measurement data; performing a regression analysis on the asymmetry measurement data by fitting a linear regression model to a planar representation of asymmetry measurements for the first target structure against asymmetry measurements for the second target structure, the linear regression model not necessarily being fitted through an origin of the planar representation; and determining the overlay error from a gradient described by the linear regression model; wherein the first deliberate overlay bias is a positive overlay bias and the second deliberate overlay bias is a negative overlay bias and the first deliberate overlay bias and the second deliberate overlay bias are of equal magnitude.