Master’s Examination Committee:

There exist several mathematical models that can explain choice, confidence, and reaction time in the context of simple decision making. The assumptions these mod-els make about the accumulation of evidence used in making a decision also have implications for response reversals. This project used the expanded judgment task to test the expanded Poisson race model’s predictions of the frequency of response reversals. For the first experiment, subjects had to decide between two decks of red and green cards, one of which was predominantly red while the other was predomi-nantly green. Subjects viewed sequences of 1, 3, or 5 cards randomly drawn from one the decks, made a decision, and then viewed a further 2, 4, or 6 cards and made a confidence judgment in which they had the opportunity to reverse their decision. The results found that, qualitatively, the model fit the trends of the empirical data but tended to underestimate the average number of reversals. To examine whether the deviations from the model were due to order effects, a second experiment was run in which subjects only viewed 3 cards initially and 2 cards in the end, but every possibl