This study attempted to replicate and extend the study of Doherty, Mynatt, Tweney, and Schiavo (1979), which introduced what is here called the Bayesian conditionals selection paradigm. The present study used this paradigm (and a script similar to that used by Doherty et al.) to explore confirmation bias and related errors that can appear in both search and integration in probability revision. Despite selection differences and weak manipulations, this study provided information relevant to four important questions.
First, by asking participants to estimate the values of the conditional probabilities they did not learn, this study was able to examine the use of “intuitive conditionals”. This study found evidence that participants used intuitive conditionals and that their intuitive conditionals were affected by the size of the actual conditionals.
Second, by examining both phases in the same study, this study became the first to look for inter-phase interactions. A strong correlation was found between the use of focal search strategies and focal integration strategies (r=.81, p<.001). However, when the sample was limited to participants who selected at least one probability conditioned on each hypothesis, the relationship was near random (r=.02).
Third, this study was the first in the Bayesian conditionals selection paradigm to provide a basis for selecting information that would be expected to confirm rather than disconfirm the focal hypothesis. No support was found for predictor selection bias, but a nearly significant interaction was found between Attention and Motivation (p=.07) in applying Bayes’ theorem only to information favoring the focal hypothesis.
Fourth, this study was the first to have a single normative posterior probability, against which participants’ posterior probability estimates could be compared to yield a quantitative measure of confirmation bias. This permitted measuring the confirmation bias uniquely contributed by each phase, assuming the participant was normative in the other phase. Focused attention was found to increase confirmation bias in the integration phase ( p=.03) but not in the search phase.
These last three findings challenge the field’s assumption that the search and integration phases can be examined separately, and call for reinterpretation of research done on only one phase.