Raising the curtain on the gray region

The gray region in EPA Document QA/G-4 is defined as the range of possible parameter values near the action level where the cost of determining that the alternative condition is true outweighs the expected consequences of a decision error. The gray region is also described as a range of true parameter values within the alternative condition near the action level where it is "too close to call." EPA Document QA/G-4HW states that during the planning stage the action level is based on an ideal decision rule, while during the assessment stage an operational decision rule is used. This paper analyzes the factors that define the gray region and the action level, including the errors of the first kind (a) and second kind (beta) and the number of samples taken to determine the mean result. The relationship between the Decision Performance Curve presented in EPA QA/G-4 and the statistical power curve is also discussed. The statistically derived critical level is identified as the concentration of importance for decision-making. The action level is defined in terms of the critical level so that its value is consistent for decisions made during both planning (a priori decisions) and assessment (a posteriori decisions).