Methods and apparatus for estimating an unknown value of at least one of a plurality of sets of data, each set of data including a plurality of values indicative of radiation diffracted and/or reflected and/or scattered by one or more features fabricated in or on a substrate, wherein the plurality of sets of data include at least one known value, and wherein at least one of the plurality of sets of data includes an unknown value, the apparatus including a processor to estimate the unknown value of the at least one set of data based on: the known values of the plurality of sets of data, a first condition between two or more values within a set of data of the plurality of sets of data, and a second condition between two or more values being part of different sets of data of the plurality of the sets of data.