The tendency to base a prediction of the frequency of an event on how easily one example comes to mind.
When a story (anecdote, testimonial) like "I know an Aussie who..." is used to prove a claim, the availability bias has kicked in. Because an example is "mentally available," we think it is representative of the whole, even if it is a glaring exception. Essentially the availability bias operates on the notion that "If you can think of it, it must be important." It thrives on what is readily available in memory, what is vivid, unusual, or emotionally charged. Two examples follow:
• Someone says to a group of friends, "People who drive red cars get more speeding tickets." The group agrees because they all know Jason who drives a red Porsche and frequently gets nabbed for speeding. The reality is, he has a history of driving fast in every car he's owned since high school. His previous cars were blue, silver and black.
• Tom told me that cigarettes—he smoked Camels without a filter and marijuana—aren't unhealthy because his grandfather smoked two packs a day and lived to be 93. "He passed away with a cigarette in his hands." Tom died of lung cancer at 64. He bet his life on his story.
"The availability bias is present in the minds of everyone from young children to professional economists. Research shows that even formal education through the PhD level has only marginal effects on this bias," says John Ray, a decision-science major at Carnegie-Mellon University, in Skeptical Inquirer magazine (March/April 2009, p. 41)
The availability bias was discovered in 1974 by Daniel Kahnemann and Amos Tversky who said, "There are situations in which people assess the probability of an event by the ease with which instances or occurrences can be brought to mind."