What should a State consider when selecting the Null Hypothesis?
States should also highlight policy decisions implicit in the statistical analysis employed. For example, if hypothesis testing is used, the State should explain why it chose either “meeting WQS” or “not meeting WQS” as the null hypothesis. Starting with the assumption that a water is “healthy” when employing hypothesis testing means that a water will be identified as impaired, and placed in Category 4 or 5, only if substantial amounts of credible evidence to refute the presumption that the water is not impaired are brought to light. As EPA explained in draft Appendices C and D of CALM, which “null hypothesis” is selected may create different incentives regarding support for additional ambient monitoring. If the null hypothesis is “meeting standards,” there was no previously data on the water, and no additional existing and readily available data and information are collected, then the “null hypothesis” cannot be rejected, and the water would not be placed in Category 4 or 5. In this s
