Scalar Expectancy Theory, or SET, is an important model of animal timing behavior credited to John Gibbon in the late 1900s. The idea of SET is that animals have an internal clock and particular memory and decision-making processes, which explains why their behavior can be temporally controlled using fixed-interval reinforcement. The scalar part of SET comes from how animals compare the current time (held in their working memory) to the expected time (held in their reference memory). When the ratio is small enough, the animal performs the behavior. When the ratio is big enough, the animal stops doing the behavior. This ratio allows for the observation that animal timing accuracy is relative to the size of the interval being timed.
Although Gibbon’s theory was intended to apply only to animals, John Wearden claimed that SET could also be applied to humans. However, human behavior has much more variability than animal behavior does, mostly due to our attentional allocation, so this application is somewhat debated.
While SET was one of the first models of timing, it is not the only one: there are many alternative models of timing, some based on SET and some completely different. When using SET, it is important to keep in mind that it might not work for all durations, and some psychologists argue that it is inconsistent when it comes to explaining the location of temporal indifference point in temporal bisection procedure.