1. A method for monitoring a manufacturing operation, said method comprising: performing multiple runs of a processing operation upon semiconductor substrates over time, said processing operation having a measurable impact on a characteristic of said semiconductor substrate; measuring said characteristic at a plurality of sites on said corresponding semiconductor substrate for each said run; identifying an outlier site of said plurality of sites wherein said measured characteristic has the largest or smallest average value over said runs; and generating a comparative index comparing a value of said measured characteristics of said outlier site to a value of said measured characteristics of other sites of said plurality of sites, for each said run.