A method of controlling the lithography process used to fabricate patterns on layers of a semiconductor wafer is disclosed. The method includes providing at least two scatterometry targets, each target having a first pattern formed in an upper layer substantially aligned with a second pattern formed in a lower layer. The targets are optically inspected. A theoretical model of each target is created, with each model including a plurality of unknown parameters defining the target and wherein at least one of the parameters is common to each of the targets. A regression analysis is performed wherein the measured optical response of the targets is compared to calculated optical responses generated by varying the values of the parameters applied to the model. During the regression analysis, a common value for the common parameter is maintained. The results are used to control the lithography process.