A method of grouping data associated with substrates undergoing a process step of a manufacturing process is disclosed. The method includes obtaining first data associated with substrates before being subject to the process step and obtaining a plurality of sets of second data associated with substrates after being subject to the process step, each set of second data being associated with a different value of a characteristic of the first data. A distance metric is determined which describes a measure of distance between the sets of second data, and the second data is grouped based on a property of the distance metric.