A heuristic is a rule of thumb that serves as a mental shortcut for judgments and decisions. For example, many people are afraid of becoming victims of a crime, often more so than is justified by official crime statistics. One possible explanation for the imbalance between fear and risk is that people overestimate the prevalence of violent crime because such crimes are exhaustively covered and sensationalized by the media. This coverage enhances the availability of violent crimes in a person's memory, causing the frequency of violent crime to be overestimated. Availability is defined as the ease with which instances of a class, such as “violent crimes,” come to mind, and it is often used as a heuristic in judging frequencies of that instance.