This chapter describes the use of basic arithmetic and probably some concentrated effort, it's likely that can be a better thinker for having worked through the problems. Probability is the study of likelihood and uncertainty. There is a large research literature documenting the fact that most people are biased in their assessment of probabilities. It is not a surprising finding given that It can only use probabilities to understand events "in the long run" and most of the everyday experiences are based on short-run observations. In an experimental investigation of the overconfidence phenomenon, people were asked to provide answers with a specified degree of confidence to factual questions. Subjective probability refers to personal estimates of the likelihood of events. Consider Dreman's summary of a large body of research on this effect: "The tendency to underestimate or altogether ignore past probabilities in making a decision is undoubtedly the most significant problem of intuitive predictions".