In a production line, a cluster tool having a plurality of substantially identical process modules and a metrology tool includes a control unit that allows one to receive, store and process information that indicates in which process module which substrates have been processed and which selects, on the basis of the process information, which substrate has to be subjected to a measurement. Advantageously, the substrates are selected so that each process module is represented by a corresponding substrate to be measured in order to reliably monitor the process quality of each process module.