The present disclosure is directed to a method of determining one or more focus values for a lithographic scanner. An optical signal including at least a first variable and a second variable is detected by an optical analysis system from at least one test sample for a plurality of programmed focus error values. A first variable value showing sensitivity to focus is selected based upon a corresponding responsiveness of the second variable to change of focus and/or a corresponding linearity of raw focus with respect to the programmed focus error. At least one focus value for the lithographic scanner is determined based upon at least one determined raw focus value corresponding to the selected first variable value.