Psychophysics is concerned with how we perceive the physical stimuli impinging upon our senses. The branch of psychophysics that deals with the perception of sound is psychoacoustics. The former is sensory capability or sensitivity, and the latter is response proclivity. For the most part, we are interested in sensory capability. In the method of limits, the stimulus is under the investigator's control and the subject simply responds after each presentation. Suppose we are interested in the absolute sensitivity or threshold for a particular sound. The method of constant stimuli involves the presentation of various stimulus levels to the subject in random order. Unlike the methods of limits and adjustments, the method of constants is a nonsequential procedure. Category or categorical rating scales are used primarily for making loudness measurements. Although most psychoacoustic studies of loudness now use magnitude estimation and/or loudness balancing methods, category rating scales are used extensively in clinical audiology, particularly in assessments related to hearing aids.