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The availability heuristic is a mental shortcut that relies on immediate examples that come to mind. The availability heuristic operates on the notion that if something can be recalled, it must be important. Subsequently, people tend to heavily weigh their judgments toward more recent information, making new opinions biased toward that latest news.  Further, the availability of consequences associated with an action is positively related to perceptions of the magnitude of the consequences of that action. In other words, the easier it is to recall the consequences of something, the greater we perceive these consequences to be. Finally, people not only consider what they recall in making a judgment but also use the ease or difficulty with which that content comes to mind as an additional source of information. Most notably, they only rely on the content of their recall if its implications are not called into question by the difficulty that they experience in bringing the relevant material to mind. 
The following are three heuristic principles that people rely on in situations of uncertainty. These principles reduce the complex task of assessing probabilities and predicting values to simpler judgmental operations.
- Representativeness, which is usually employed when people are asked to judge the probability that an object or event A belongs to class or process B.
- Availability of instances or scenarios, which is often employed when people are asked to assess the frequency of a class or the plausibility of a particular development.
- Adjustment from an anchor, which is usually employed in numerical prediction when a relevant value is available. 
Sometimes the heuristic is beneficial, but the frequencies that events come to mind are usually not accurate reflections of their actual probability in real life. For example, if a student is asked whether her college had more students from Colorado or more from California, her answer would probably be based on the personal examples she is able to recall.
Overview and history
When faced with the difficult task of judging probability or frequency, people use a limited number of strategies, called heuristics, to simplify these judgements. One of these strategies, the availability heuristic, is the tendency to make a judgement about the frequency of an event based on how easy it is to recall similar instances. In 1973, Amos Tversky and Daniel Kahneman first studied this phenomenon and labeled it the Availability Heuristic. The availability heuristic is an unconscious process that operates on the notion that, "if you can think of it, it must be important."  In other words, how easily an example can be called to mind is related to perceptions about how often this event occurs. Thus, people tend to use a readily accessible attribute to base their beliefs about a relatively distant concept.
In an experiment to test this heuristic, Tversky and Kahneman presented participants with four lists of names: two lists with the names of 19 famous women and 20 less famous men, and two lists with the names of 19 famous men and 20 less famous women. The first group was asked to recall as many names as possible and the second group was asked to estimate which class of names was more frequent: the names from famous celebrities or less famous celebrities. The names from the famous celebrities were most easily recalled compared to those from the less famous celebrities, and despite the fact that more people in society shared names with the less famous celebrities than the more famous ones, the majority of the participants incorrectly judged that the famous names occurred more often. While the availability heuristic is an effective strategy in many situations, when judging probability the availability heuristic can lead to systematic errors.
In a study by Schwarz et al., participants were asked to describe either six or twelve examples of assertive respective unassertive behavior, or vice versa. Participants were later asked to rate their own assertiveness. The results indicated that participants rated themselves as more unassertive after describing six examples for the unassertive behavior condition, and conversely rated themselves as less assertive after describing twelve examples of the unassertive behavior condition. The study reflected that the recalled content (was qualified) by the ease with which the content could be brought to mind (it was easier to recall 6 examples than 12).
In another study, subjects were asked, "If a random word is taken from an English text, is it more likely that the word starts with a K, or that K is the third letter?" Most English-speaking people could immediately think of many words that begin with the letter "K" (kangaroo, kitchen, kale), but it would take a more concentrated effort to think of any words where "K" is the third letter (acknowledge, ask). Results indicated that participants overestimated the number of words that began with the letter "K" and underestimated the number of words that had "K" as the third letter. Researchers concluded that people answer questions like these by comparing the availability of the two categories and assessing how easily they can recall these instances. In other words, it is easier to think of words that begin with "K", than words with "K" as the third letter. Thus, people judge words beginning with a "K" to be a more common occurrence. In reality, however, a typical text contains twice as many words that have "K" as the third letter than "K" as the first letter. There are three times more words with "K" in the third position than words that begin with "K".
Chapman (1967) described a bias in the judgment of the frequency with which two events co-occur. This demonstration showed that the co-occurrence of paired stimuli resulted in participants overestimating the frequency of the pairings. To test this idea, participants were given information about several hypothetical mental patients. The data for each patient consisted of a clinical diagnosis and a drawing made by the patient. Later, participants estimated the frequency with which each diagnosis had been accompanied by various features of the drawing. The subjects vastly overestimated the frequency of this co-occurrence (such as suspiciousness and peculiar eyes). This effect was labeled the illusory correlation. Tversky and Kahneman suggested that availability provides a natural account for the illusory-correlation effect. The strength of the association between two events could provide the basis for the judgment of how frequently the two events co-occur. When the association is strong, it becomes more likely to conclude that the events have been paired frequently. Strong associations will be thought of as having occurred together frequently.
Research in 1992 used mood manipulation to influence the availability heuristic by placing participants into a sad mood condition or a happy mood condition. People in the sad mood condition recalled better than those in the happy mood condition, revealing that the power of the availability heuristic changes in certain conditions.
- A person claims to a group of friends that those who drive red cars receive more speeding tickets. The group agrees with the statement because a member of the group drives a red car and frequently receives speeding tickets. The reality could be that because he drives fast, he receives speeding tickets regardless of his car's color. Even if statistics show that fewer speeding tickets are given to red cars than to cars of other colors, he is an available example, which makes the statement seem more plausible.
- Where an anecdote ("I know a Peruvian man who ...") is used to "prove" an entire proposition or to support a bias, the availability heuristic is in play. In these instances the ease of imagining an example or the vividness and emotional impact of that example becomes more credible than actual statistical probability. Because an example is easily brought to mind or mentally "available," the single example is considered representative of the whole rather than as just an anecdotal example in a range of data. An example is when a person argues that cigarette smoking is not unhealthy because his grandfather smoked three packs of cigarettes each day and lived to be 100 years old. (The grandfather's health could be an exception to the rule.)
- A person sees several news stories about cats leaping out of tall trees and surviving, so he believes that cats must be robust to long falls. However, these kinds of news reports are far more uncommon than reports where a cat falls out of the tree and dies, which could be more common.
- A recent newspaper subscriber might compare the number of newspapers delivered versus those that were not delivered in order to calculate newspaper delivery failure. In this case, the calculation of delivery failure depends on the number of incidents recalled. However, it will be hard to recall all specific instances if the subscriber is trying to recall all newspaper deliveries over an extensive period of time.
- After seeing many news stories of home foreclosures, people may judge that the likelihood of this event is greater. This may be true because it is easier to think of examples of this event.
After seeing news stories about child abductions, people may judge that the likelihood of this event is greater. Media coverage can help fuel a person's example bias with widespread and extensive coverage of unusual events, such as homicide or airline accidents, and less coverage of more routine, less sensational events, such as common diseases or car accidents. For example, when asked to rate the probability of a variety of causes of death, people tend to rate "newsworthy" events as more likely because they can more readily recall an example from memory. Moreover, unusual and vivid events like homicides, shark attacks, or lightning are more often reported in mass media than common and unsensational causes of death like common diseases.
For example, many people think that the likelihood of dying from shark attacks is greater than that of dying from being hit by falling airplane parts, when more people actually die from falling airplane parts. When a shark attack occurs, the deaths are widely reported in the media whereas deaths as a result of being hit by falling airplane parts are rarely reported in the media.
In a 2010 study exploring how vivid television portrayals are used when forming social reality judgments, people watching vivid violent media gave higher estimates of the prevalence of crime and police immorality in the real world than those not exposed to vivid television. These results suggest that television violence does in fact have a direct causal impact on participants' social reality beliefs. Repeated exposure to vivid violence leads to an increase in people's risk estimates about the prevalence of crime and violence in the real world. Counter to these findings, researchers from a similar study argued that these effects may be due to effects of new information. Researchers tested the new information effect by showing movies depicting dramatic risk events and measuring their risk assessment after the film. Contrary to previous research, there were no effects on risk perception due to exposure to dramatic movies.
Researchers examined the role of cognitive heuristics in the AIDS risk-assessment process. 331 physicians reported worry about on-the-job HIV exposure, and experience with patients who have HIV. By analyzing answers to questionnaires handed out, researchers concluded Availability of AIDS information did not relate strongly to perceived risk.
Participants in a 1992 study read case descriptions of hypothetical patients who varied on their sex and sexual preference. These hypothetical patients showed symptoms of two different diseases. Participants were instructed to indicate which disease they thought the patient had and then they rated patient responsibility and interactional desirability. Consistent with the availability heuristic, either the more common (influenza) or the more publicized (AIDS) disease was chosen.
Business and economy
One study sought to analyze the role of the availability heuristic in financial markets. Researchers defined and tested two aspects of the availability heuristic:
- Outcome Availability – availability of positive and negative investment outcomes, and
- Risk Availability – availability of financial risk
On days of substantial stock market moves, abnormal stock price reactions to upgrades are weaker, than those to downgrades. These availability effects are still significant even after controlling for event-specific and company-specific factors.
Similarly, research has pointed out that under the availability heuristic, humans are not reliable because they assess probabilities by giving more weight to current or easily recalled information instead of processing all relevant information. Since information regarding the current state of the economy is readily available, researchers attempted to expose the properties of business cycles to predict the availability bias in analysts' growth forecasts. They showed the availability heuristic to play a role in analysis of forecasts and influence investments because of this.
In effect, investors are using availability heuristic to make decisions and subsequently, may be obstructing their own investment success. An investor's lingering perceptions of a dire market environment may be causing them to view investment opportunities through an overly negative lens, making it less appealing to consider taking on investment risk, no matter how small the returns on perceived "safe" investments. To illustrate, Franklin Templeton's annual Global Investor Sentiment Survey 1 asked individuals how they believed the S&P 500 Index performed in 2009, 2010 and 2011. 66 percent of respondents stated that they believed the market was either flat or down in 2009, 48 percent said the same about 2010 and 53 percent also said the same about 2011. In reality, the S&P 500 saw 26.5 percent annual returns in 2009, 15.1 percent annual returns in 2010 and 2.1 percent annual returns in 2011, meaning lingering perceptions based on dramatic, painful events are impacting decision-making even when those events are over. 
Additionally, a study by Hayibor and Wasieleski found that the availability of others who believe that a particular act is morally acceptable is positively related to others' perceptions of the morality of that act. This suggests that availability heuristic also has an effect on ethical decision making and ethical behavior in organizations.
A study done by Craig R. Fox provides an example of how availability heuristics can work in the classroom. In this study, Fox tests whether difficulty of recall influences judgment, specifically with course evaluations among college students. In his study he had two groups complete a course evaluation form. He asked the first group to write two recommended improvements for the course (a relatively easy task) and then write two positives about the class. The second group was asked to write ten suggestions where the professor could improve (a relatively difficult task) and then write two positive comments about the course. At the end of the evaluation both groups were asked to rate the course on a scale from one to seven. The results showed that students asked to write ten suggestions (difficult task) rated the course less harshly because it was more difficult for them to recall the information.[why?] Students asked to do the easier evaluation with only two complaints had less difficulty in terms of availability of information, so they rated the course more harshly.
The media usually focuses on violent or extreme cases, which are more readily available in the public's mind. This may come into play when it is time for the judicial system to evaluate and determine the proper punishment for a crime. In one study, respondents rated how much they agreed with hypothetical laws and policies such as "Would you support a law that required all offenders convicted of unarmed muggings to serve a minimum prison term of two years?" Participants then read cases and rated each case on several questions about punishment. As hypothesized, respondents recalled more easily from long-term memory stories that contain severe harm, which seemed to influence their sentencing choices to make them push for harsher punishments. This can be eliminated by adding high concrete or high contextually distinct details into the crime stories about less severe injuries.
A similar study asked jurors and college students to choose sentences on four severe criminal cases in which prison was a possible but not an inevitable sentencing outcome. Respondents answering questions about court performance on a public opinion formulated a picture of what the courts do and then evaluated the appropriateness of that behavior. Respondents recalled from public information about crime and sentencing. This type of information is incomplete because the news media present a highly selective and non-representative selection of crime, focusing on the violent and extreme, rather than the ordinary. This makes most people think that judges are too lenient. But, when asked to choose the punishments, the sentences given by students were equal to or less severe than those given by judges. In other words, the availability heuristic made people believe that judges and jurors were too lenient in the courtroom, but the participants gave similar sentences when placed in the position of the judge, suggesting that the information they recalled was not correct.
Researchers in 1989 predicted that mock jurors would rate a witness to be more deceptive if the witness testified truthfully before lying than when the witness was caught lying first before telling the truth. If the availability heuristic played a role in this, lying second would remain in jurors' minds (since it was more recent) and they would most likely remember the witness lying over the truthfulness. To test the hypothesis, 312 university students played the roles of mock jurors and watched a videotape of a witness presenting testimony during a trial. Results confirmed the hypothesis, as mock jurors were most influenced by the most recent act.
|This section may require copy-editing for clearer language. (October 2013)|
Some researchers have suggested that perceived causes or reasons for an event, rather than imagery of the event itself, influence probability estimates. Evidence for this notion stems from a study where participants either imagined the winner of the debate, or came up with reasons for why Ronald Reagan or Walter Mondale would win the 1984 U.S. Presidential Candidate debate. The results of this study explained that imagining Reagan or Mondale winning the debate had no effect on predictions of who would win the debate. However, imagining and considering reasons for why Reagan or Mondale would win the debate did significantly affect predictions.
Other psychologists argue that the classic studies on the availability heuristic are vague and do not explain the underlying processes. For example, in the famous Tversky and Kahneman study, Wanke et al. believe that this differential ease of recall, may alter subjects' frequency estimates in two different ways. In one way, as the availability heuristic suggests, the subjects may use the subjective experience of ease or difficulty of recall as a basis of judgment. Researchers also assert that if this is done, they would predict a higher frequency if the recall task is experienced as easy rather than difficult. In a contrasting scenario, researchers suggest that the subjects may recall as many words of each type as possible within the time given to them and may base their judgment on the recalled sample of words. If it is easier to recall words which begin with a certain letter, these words would be over-represented in the recalled sample, again producing a prediction of higher frequency. In the second scenario the estimate would be based on recalled content rather than on the subjective experience of ease of recall.
Some researchers have shown concern about confounding variables in the original Tversky and Kahneman study. Researchers question if the participants recalling celebrity names were basing frequency estimates on the amount of content recalled or on the ease of recall. Some researchers suggest that the design of the earlier experiment was flawed and did not actually determine how the availability heuristic works.
Recent research has provided some evidence that the availability heuristic is only one of many strategies involved in frequency judgment. Future research should attempt to incorporate all these factors.
- Affect heuristic
- Agenda-setting theory
- Anecdotal evidence
- Anecdotal value
- Attribute substitution
- Gambler's fallacy
- Illusory correlation
- List of biases in judgment and decision making
- Misleading vividness
- Processing fluency
- Representativeness heuristic
- Texas sharpshooter fallacy
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- How Belief Works - an article on the origins of the availability bias.