The heuristic is based on one's estimation of the frequency or likelihood of an event based on how easy it is to recall relevant information from memory or how easy it is to think of related examples.

Availability Heuristic

A heuristic is a mental shortcut that we often use to make judgments and decisions. Availability heuristic is based on the principle that the first things come to our mind when evaluating certain topics, events, and methods, must be the based on the more common occurrences, or must be more important than other available alternatives which do not easily come to mind.

The problem with this type of mental shortcut, however, is that it can easily be influenced or biased towards vivid, dramatic events, or recent news and information, especially those sensationalized by the media. 

For example, people might take more precaution against murder and car accidents compared to illnesses such as diabetes and stomach cancer, as the two former are more sensationalized or are often reported in the news than the latter two. However, statistics show that in America, these illness actually take twice as much lives than murder and car accidents (source: http://study.com/academy/lesson/availability-heuristic-examples-definition-quiz.html).