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.