Prior to 2001, employer business activity was classified according to the Standard Industrial Classification (SIC) system. In 2001, a conversion from the SIC to the North American Industry Classification System (NAICS) occurred. These data series are not comparable. Also, in order to ensure high quality data, the QCEW Unit updates, if necessary, the NAICS industry code, geographic location, and ownership classifications of employers on a continuous basis. These non-economic changes are introduced in the first quarter of each year. Due to these changes in industry and/or geographic location, QCEW data are often not comparable as a time series.
Metropolitan Statistical Areas (MSAs) are also redefined from time to time. It is important to take these redefinitions into account when comparing QCEW data for different time periods.