The argument against mathematical guides to aid a fact finder is that the trial is not only a search for truth, but also a social ritual which supports certain values and helps litigants and the society accept the judgments of courts. Therefore, the more formal mathematical processing departs from intuition, the more it should be eschewed by the courts. However, in many contexts decisionmakers' initutive, common sense judgments depart markedly from the actual probabilities. People use simplifying operations, called 'heuristics,' to reduce the complexity of information which must be integrated to yield a decision. These simplifying strategies often lead to errors in judgment. Furthermore, people tend to be overconfident in their judgments because previous information is reinterpreted in light of the new and two sets of information are integrated into a coherent whole. Moreover, in a criminal trial, jurors are first given the 'answer' -that is, the defendant -- and the evidence is provided afterwards. Such arrangement is especially prone to hindsight. On the other hand, the mathematical model of a person's own decision policies is more accurate because it consistently applies the same logic. Furthermore, statistical data need not be regarded as so overwhelming and, therefore, prejudicial, since people do not process probabilistic information well; faced with both anecdotal and statistical evidence, they tend to ignore the latter. Moreover, the assumption that case-specific information is qualitatively different from base-rate information is erroneous. From the viewpoint of a disinterested fact finder all information is indirect and imperfectly credible. Formal mathematical models also have room for errors, but properly employed and developed, they err less than intuitive techniques. Three fundamental heuristics involved in making probabilistic judgments (representativeness, availability, and anchoring and adjustment) are described. Relevant psychological research is analyzed and the evaluation of witness credibility is discussed. Limited statistical data, footnotes, and over 80 references are included. A list of cases cited is also provided.