An Introduction to the Diffusion Model of Decision Making

  • Philip L. Smith
  • Roger Ratcliff


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 links evidence accumulation to processes of perception, attention, and memory, and in neuroscience, to neural firing rates in the oculomotor control system in monkeys performing saccade-to-target decision tasks.


Diffusion process Random walk Decision-making Response time Choice probability 


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Copyright information

© Springer Science+Business Media, LLC 2015

Authors and Affiliations

  • Philip L. Smith
    • 1
  • Roger Ratcliff
    • 2
  1. 1.Melbourne School of Psychological SciencesThe University of MelbourneMelbourneAustralia
  2. 2.Department of PsychologyThe Ohio State UniversityColumbusUSA

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