Loosely defined or informal rules often arrived at through experience or trial and error that make assessments and decisions (e.g., gastrointestinal complaints that wake patients up at night are unlikely to be benign in nature). Heuristics provide cognitive shortcuts in the face of complex situations, and thus serve an important purpose. Unfortunately, they can also turn out to be wrong, with frequently used heuristics often forming the basis for the many cognitive biases, such as anchoring bias, availability bias, confirmation bias, and others, that have received attention in the literature on diagnostic errors and medical decision making.