The most important changes that have taken place in behaviorism since John B. Watson's 1913 article are the introduction of the intervening variable approach and the understanding that psychology is both an experimental and a psychometric science. The only observables available to psychology are stimuli and response. (SLD)

Article is a response to the contention that the inhibition associated with S is nonmonotonic with respect to non-reinforced trails. (Author)

Sequential effects in 2CRT are due to the structure of the sequence of stimuli, rather than due to local response biasing resulting from the sequence of required responses. (Author)

The article by W. K. Estes marks a turning point in the mathematical learning theory movement. The central constructs were stimulus variability, stimulus sampling, and stimulus response association by contiguity, in a framework enabling prediction of response probability and latency. (SLD)