1. A method comprising: receiving reference values of one or more parameters of interest of one or more actual device structures repeatedly fabricated over a range of values of one or more process variables, wherein the reference values are measured by a reference metrology system; receiving a first amount of measurement data associated with measurements of one or more metrology targets disposed nearby the one or more actual device structures, wherein the first amount of measurement data is derived from measurements performed by at least one optical metrology technique; and training a measurement model based on the first amount of measurement data and the reference values of the one or more parameters of interest.