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; providing a first amount of illumination light to one or more metrology targets disposed nearby the one or more actual device structures; detecting an amount of light from the one or more metrology targets in response to the first amount of illumination light provided to the one or more metrology targets, the detected amount of light comprising a first amount of optical measurement data; receiving the first amount of measurement data, wherein the first amount of measurement data is derived from measurements performed by at least one optical metrology system that is different from the reference metrology system; 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.