Evidence and Uncertainty in Human Reasoning

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Although some decisions are made on the basis of simple directevaluation of "gut" feelings, or the utilities of the variousalternatives, many decisions involve conflicting feelings; and theconflict-resolution process often takes into account evidence aboutsome uncertain events.A typical example would be the conflict faced by a college student whohas an important exam the next day but wants to quit studying tospend some time with friends. In the process of resolving thisconflict, many students consider the uncertain event that they will dowell in the exam even if they stop studying now. Evidence relevant tothis event includes how difficult the exam is expected to be and howmuch preparation the student has done all semester in this course. Ifthe exam is only somewhat important, and the evidence leads the studentto feel pretty sure of doing well without further studying, the studentwill probably decide to quit studying. Even if the exam is highlyimportant, a student who is very sure of doing well will probablyquit studying.For some decisions, part of the evidence consists of estimatedprobabilities derived from scientific models. A physician mayconsider estimated probabilities of recurrence of cancer beforerecommending therapy, or a homeowner may take account of earthquakeprobabilities in deciding whether to reinforce the foundation.The present research is based on a recently developed theory ofdecision-making under uncertainty. The core idea is that peoplecategorize uncertain events (e.g., "pretty sure" or "very sure"of doing well on the exam), and that such categorization is based onvarious kinds of evidence (sometimes including probabilitiesestimated from scientific models).The research has two main branches. One focusses on decisionsituations involving conflict. We study how decision rules change,depending on value of goals (for example, a student may use a"pretty sure" rule for a somewhat important exam, but a "very sure"rule for a highly important one) and how evidence is used tocategorize the uncertain events. Our preliminary findings suggestthat different evidence weightings are used for different abstractcategories of uncertainty. This part of the research also examinespeople's justifications for their decisions, in order to understandbetter how justifications relate to actual decision rules underlyingchoices.The second branch of the research focusses on evidence judgment, inisolation from the ultimate decision--for example, the judgmentsthat a detective or a scientist might use in forming or confirminghypotheses. We are interested in the difference between initiallygenerating a hypothesis and confirming it; in how people react toconflicting evidence; in how people use base rates and relativelikelihoods as evidence; and in the process of selecting relevantevidence when it is obscured by irrelevancies.To summarize, the major goals of the research include testing anddeveloping a rather new theory of decision making; studying howscientific probability estimates can be used in policy decisions;and delineating the differences between exploratory use of evidencein formulating hypotheses versus hypothesis confirmation.