The performance of the MMIs was tested with a validation dataset.
We conducted testing and validation in three steps: first, we test MMI sensitivity using a criterion to determine if the MMI scores developed from the calibration dataset could distinguish reference from impaired conditions in the validation dataset (namely, percent of sites correctly classified).
The 25th percentile of MMI scores at reference sites in each ecoregion from the calibration dataset was used as a critical value (Stoddard et al., 2008)