We use the availability heuristic to estimate the frequency of specific events. For example, how often are people killed by mass murderers? Because higher frequency events are more likely to occur at any given moment, we also use the availability heuristic to estimate the probability that events will occur. For example, what is the probability that I will be killed by a mass murderer tomorrow?
We are especially reliant upon the availability heuristic when we do not have solid evidence from which to base our estimates. For example, what is the probability that the next plane you fly on will crash? The true probability of any particular plane crashing depends on a huge number of factors, most of which you're not aware of and/or don't have reliable data on. What type of plane is it? What time of day is the flight? What is the weather like? What is the safety history of this particular plane? When was the last time the plane was examined for problems? Who did the examination and how thorough was it? Who is flying the plane? How much sleep did they get last night? How old are they? Are they taking any medications? You get the idea.
The chances are excellent that you do not have access to all or even most of the information needed to make accurate estimates for just about anything. Indeed, you probably have little or no data from which to base your estimate. Well, that's not exactly true. In fact, there is one piece that evidence that you always have access to: your memory. Specifically, how easily can you recall previous incidents of the event in question? The easier time we have recalling prior incidents, the greater probability the event has of occurring -- at least as far as our minds are concerned. In a nutshell, this is the availability heuristic.
Although there are many problems associated with the availability heuristic, perhaps the most concerning one is that it often leads people to lose sight of life's real dangers. Psychologist Gerd Gigerenzer, for example, conducted a fascinating study that showed in the months following September 11, 2001, Americans were less likely to travel by air and more likely to instead travel by car. While it is understandable why Americans would have been fearful of air travel following the incredibly high profile attacks on New York and Washington, the unfortunate result is that Americans died on the highways at alarming rates following 9/11. This is because highway travel is far more dangerous than air travel. More than 40,000 Americans are killed every year on America's roads. Fewer than 1,000 people die in airplane accidents, and even fewer people are killed aboard commercial airlines.
Consider, for example, that the 2009 budget for homeland security (the folks that protect us from terrorists) will likely be about $50 billion. Don't get us wrong, we like the fact that people are trying to prevent terrorism, but even at its absolute worst, terrorists killed about 3,000 Americans in a single year. And less than 100 Americans are killed by terrorists in most years. By contrast, the budget for the National Highway Traffic Safety Administration (the folks who protect us on the road) is about $1 billion, even though more than 40,000 people will die this year on the nation's roads. In terms of dollars spent per fatality, we fund terrorism prevention at about $17,000,000/fatality (i.e., $50 billion/3,000 fatalities) and accident prevention at about $25,000/fatality (i.e., $1 billion/40,000 fatalities).
(Nod to MAG, with thanks)