An Introduction to the Diffusion Model of Decision-Making

The diffusion model assumes that two-choice decisions are made by accumulating successive samples of noisy evidence to a response criterion. The model has a pair of criteria that represent the amounts of evidence needed to make each response. The time taken to reach criterion determines the decision time and the criterion that is reached first determines the response. The model predicts choice probabilities and the distributions of response times for correct responses and errors as a function of experimental conditions such as stimulus discriminability, speed-accuracy instructions, and manipulations of relative stimulus frequency, which affect response bias. This chapter describes the main features of the model, including mathematical methods for obtaining response time predictions, methods for fitting it to experimental data, including alternative fitting criteria, and ways to represent the fit to multiple experimental conditions graphically in a compact way. The chapter concludes with a discussion of recent work in psychology that generalizes the diffusion model to continuous outcome decisions, in which responses are made on a continuous scale rather than categorically.