A statistical process control method for monitoring and controlling semiconductor manufacturing processing operations is provided. For a chosen processing operation, multiple measurement sites are used to generate data of a measurable characteristic that is impacted by and associated with the processing operation. The data from the sites is compared over time and one or more outlier sites are identified. The outlier sites are the sites at which the data values are most divergent from the rest of the data. Algorithms are used to mathematically compare the outlier sites to the other sites to produce a comparative index. The comparative index is monitored graphically or otherwise to identify changes in the processing operation, and corrective actions are taken.