Burrhus Frederic Skinner was a highly influential American psychologist, author, inventor, advocate for social reform and poet. He was the Edgar Pierce Professor of Psychology at Harvard University from 1958 until his retirement in 1974. He invented the operant conditioning chamber, innovated his own philosophy of science called Radical Behaviorism, and founded his own school of experimental research psychology—the experimental analysis of behavior. His analysis of human behavior culminated in his work Verbal Behavior, which has recently seen enormous increase in interest experimentally and in applied settings. He discovered and advanced the rate of response as a dependent variable in psychological research. He invented the cumulative recorder to measure rate of responding as part of his highly influential work on schedules of reinforcement. In a recent survey, Skinner was listed as the most influential psychologist of the 20th century. He was a prolific author, publishing 21 books and 180 articles.