Reference sites are used as the benchmark against which test sites are to be compared. The MPCA uses a regional reference site approach to develop and calibrate the IBI for specific regions of Minnesota. Regional reference condition is a description of the chemical, physical, or biological condition based on an aggregation of data from least-impacted sites that are representative of a waterbody type in an ecoregion, subecoregion, watershed, or political unit. Using regional reference conditions is preferable to using data from a single reference site from which to compare biosurvey results.