S. S. Stevens
The Power Law
Psychophysical Scaling and Measurement Theory
In 1934 Stanley Smith Stevens received his Ph.D. from the newly independent psychology department at Harvard, and two years later accepted a position as instructor in experimental psychology. Known professionally as “S. S. Stevens” and by his colleagues as “Smitty” in the ensuing decades, Stevens built a renowned laboratory of experimental psychology in the basement of Memorial Hall at Harvard. His own research focused on the relationship between the perceived magnitude of a stimulus and its objective physical magnitude. More generally, Stevens was a pioneer in psychoacoustics, and a major organizer of the field of experimental psychology.
In the course of his research, Stevens discovered that the most direct way of measuring the perceived intensity of a stimulus (e.g. a light, a sound, a smell, a shock) was simply to ask people to assign it a number, without putting a limit on the scale or forcing them to choose the units. (This departed from the more indirect behavioral methods that were popular at the time, such as measuring which stimuli people failed to discriminate, and assuming that all such pairs had a constant perceived magnitude.) When sensations were measured in this way, they showed an orderly relationship to the physical magnitude of the sensation, which Stevens called the Power Law: subjective sensation is a function of the physical magnitude raised to a constant power, with the exponent specific to the particular kind of stimulation (e.g., the brightness of a light or loudness of a sound). Soon hundreds of physical continua were scaled by the technique of magnitude estimation and characterized by a power law, including the attitudes toward political candidates and childbirth pain as a function of centimeters of cervical dilation. Stevens also proposed a number of units for physical signals designed to demarcate equal intervals in perceived magnitude, such as “sones” for perceived loudness.
Stevens’ Psycho-Acoustic Laboratory at Harvard trained many young psychologists, such as George Miller. In 1952, Stevens published an unprecedented and indispensable reference book, The Handbook of Experimental Psychology, which has been updated twice and is still in print and wide circulation today.
Stevens’s accomplishments also extended into the realm of quantitative data analysis. In 1946 he published an article, “On the theory of scales of measurement,” which described the characteristics and applications of four types or levels of data (nominal, ordinal, interval and ratio), universally taught to undergraduates today in courses in psychological statistics.
Stevens had a number of other unusual interests. He was an advocate of William Sheldon’s theory that personality was affected by the person’s mixture of three “somatotypes” or dimensions of body shape: endomorphic, mesomorphic, and ectomorphic. He invented many gadgets, including a short downhill ski which he passionately defended in many articles as superior to conventional skis.
Stevens spent his entire career at Harvard, shaping the department of psychology and overseeing the research and teaching of hundreds of undergraduate, graduate and post-doctoral students.
He is listed as number 52 on the American Psychological Association’s list of the 100 most eminent psychologists of the 20th century.
Eminent psychologists of the 20th century. (July/August, 2002). Monitor on Psychology, 33(7), p.29.
Harvard University Library Online Archival Search Information System (2005, March 14). Stevens, S. S. (Stanley Smith), 1906-1973. Papers of Stanley Smith Stevens : an inventory. Retrieved on December 8, 2007, from http://oasis.lib.harvard.edu/oasis/deliver/~hua07005
Miller, George A. (1975). Stanley Smith Stevens. In National Academies of Sciences: Biographical Memoirs Washington, D.C.:National Academies Press.