Two systems of measuring auditory masking thresholds by statistical psychophysical techniques are developed. The first is a succession of free-choice tests with a different level of presentation fixed for each test. In the second, the presentation level is automatically adjusted and maintained at a level close to threshold. Scoring of the two tests is developed and their interrelation is discussed. The customary use of ROC (receiver operating characteristics) notations in this type of psychophysical testing can be augmented to provide a more comprehensive measure of the over-all performance of a test subject. The term d′, developed by Tanner, Swets, and Green, provides a measure of the system under test to perform as an ideal receiver. This notation is augmented by a decision term D, which relates to the placement of the subjects' decision criterion and is independent of d′. When taken together, d′ and D define the response of the subject and show the effect of environmental changes. Alterations in environment produce changes in test results. These changes are assessed in terms of d′ and D, showing how it is possible to provide a sensitive psychophysical measure of environmental effects.