Signal detection theory and related approaches provide to distinguish between the effects of sensitivity from those of response bias. This discussion of signal detection theory has dealt primarily with the yes/no method. To recapitulate: the subject is presented with a large number of trials for each stimulus level, and a large proportion of these are actually catch trials. Just as the subject may be asked to choose between two alternatives in a single-interval trial, he might also be asked to decide which of two successive intervals contained a signal. A rating of "three" would mean "I'm not sure whether there was a signal or no signal." "Two" would indicate that there probably was no signal present, and a rating of "one" would suggest that the subject is positive that a signal was not presented. Signal detection theory is of considerable importance in psychoacoustics, because its application allows the experimenter to ferret out the effects of sensitivity and response criterion.