An article is presented configured for controlling a multiple patterning process, such as a spacer self-aligned multiple patterning, to produce a target pattern. The article comprises a test site carrying a test structure comprising at least one pair of gratings, wherein first and second gratings of the pair are in the form of first and second patterns of alternating features and spaces and differ from the target pattern by respectively different first and second values which are selected to provide together a total difference such that a differential optical response from the test structure is indicative of a pitch walking effect.