Clinical reasoning is prone to errors in judgment. Error is comprised of 2 components—bias and noise; each has an equally important role in the promulgation of error. Biases or systematic errors in reasoning are the product of misconceptions of probability and statistics. Biases arise because clinicians frequently rely on mental shortcuts or heuristics to make judgments. The most frequently used heuristics are representativeness, availability, and anchoring/adjustment which lead to the common biases of base rate neglect, misconceptions of regression, insensitivities to sample size, and fallacies of conjunctive, and disjunctive events. Bayesian reasoning is the framework within which posterior probabilities of events is identified. Familiarity with these mathematical concepts will likely enhance clinical reasoning. Noise is defined as inter or intraobserver variability in judgment that should be identical. Guidelines in medicine are a technique to reduce noise.