Method for selecting pattern reference features of a substantially irregular pattern layout

A method of determining product distribution data relating to one or more stochastic parameters characterizing a stochastic effect, for a plurality of product features within a die area of a product pattern on a patterned substrate, the method comprising:

• obtaining reference distribution data relating to a measurement of a target comprising repetitions of reference features, the reference features comprising a proper subset of the plurality of product features, or representative features thereof;
• obtaining pre-calibrated relationship data which describes relationships between the product distribution data and the reference distribution data;
• and
• determining, by a hardware computer system, product distribution data for the plurality of product features from the reference distribution data and the pre-calibrated relationship data.