A rule of thumb whereby people assess the probability of an event by the ease with which they can bring occurrences to mind. For example, which is more likely to be killed by a falling airplane part or by a shark? Shark attacks receive more publicity, so most people think they are more likely. In fact, the chance of getting killed by falling airplane parts is 30 times higher. Plous (1993, Chapter 11) discusses the availability heuristic. This heuristic can produce poor judgmental forecasts. It can be useful, however, in developing plausible scenarios.