1. A treatment system for etching a feature into a substrate, comprising: an etching system configured to etch a pattern in one or more substrates using a lithographic structure prepared in a film of light-sensitive material to radiation; a metrology system to measure a set of metrology data for said one or more substrates treated in said etching system for variations in process data for said etching system, wherein said process data comprises a gas pressure in said etching system, a flow rate of a process gas into said etching system, a power coupled to said process gas in said etching system, a time for performing an etching process in said etching system, a temperature of said one or more substrates, or a temperature of a substrate holder for supporting said one or more substrates, or any combination of two or more thereof; a data processing system configured to: obtain said set of metrology data; determine one or more essential variables for said obtained set of metrology data using multivariate analysis; obtain new metrology data measured using said metrology system; and transform said new metrology data into refined metrology data using said one or more essential variables, wherein said metrology data comprises optical metrology data, and wherein said optical metrology data comprises an optical signal resulting from illuminating one or more structures on said one or more substrates with an incident optical signal.