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The availability heuristic is a mental shortcut that relies on immediate examples that come to a given person's mind when evaluating a specific topic, concept, method or decision. The availability heuristic operates on the notion that if something can be recalled, it must be important, or at least more important than alternative solutions which are not as readily recalled. Subsequently, under the availability heuristic people tend to heavily weigh their judgments toward more recent information, making new opinions biased toward that latest news.

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 those consequences are often perceived to be. Most notably, they often 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.

Overview and history
Prior to the work of Kahneman and Tversky, the predominant view in the field of human judgment was that humans are rational actors. However, in the late 1960s and early 1970s Amos Tversky and Daniel Kahneman began work on a series of papers examining “heuristic and biases” used in judgment under uncertainty. They explained that judgment under uncertainty often relies on a limited number of simplifying heuristics rather than extensive algorithmic processing. Soon this idea spread beyond academic psychology, into law, medicine, and political science. This research questioned the descriptive adequacy of idealized models of judgment, and offered insights into the cognitive processes that explained human error without invoking motivated irrationality. One simplifying strategy people may rely on the availability heuristic, is the tendency to make a judgment about the frequency of an event based on how many similar instances are brought to mind. In 1973, Amos Tversky and Daniel Kahneman first studied this phenomenon and labeled it the "availability heuristic". An Availability heuristic is a mental shortcut that relies on immediate examples that come to a given person's mind when evaluating a specific topic, concept, method or decision. As follows, people tend to use a readily available facet to base their beliefs about a comparably distant concept.undefinedThere has been much research done with this heuristic, but studies on the issue are still questionable with regard to the underlying process. Studies illustrate that manipulations intended to increase the subjective experience of ease of recall and are also likely to affect the amount of recall. Furthermore this makes it difficult to determine if the obtained estimates of frequency, likelihood, or typicality are based on participants phenomenal experiences or on a biased sample of recalled information.

However, some textbooks have chosen the latter interpretation introducing the availability heuristic as “ones judgments are always based on what comes to mind." For example if a person is asked whether there are more words in the English Language that begin with a t or k, the person will probably be able to think of more words that begin with the letter t, concluding that t is more frequent than k.

Research
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.

In Tversky & Kahneman's first examination of availability heuristics, 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?" They argue that English-speaking people would immediately think of many words that begin with the letter "K" (kangaroo, kitchen, kale), but that it would take a more concentrated effort to think of any words in which "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. Tversky and Kahneman 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", more 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".

Explanations
Many researchers have attempted to identify the psychological process which create the availability heuristic.

Tversky and Kahneman argue that the number of examples recalled from memory is used to infer the frequency with which such instances occur. In an experiment to test this explanation, participants listened to lists of names containing either 19 famous women and 20 less famous men or 19 famous men and 20 less famous women. Subsequently, some participants were asked to recall as many names as possible whereas others were asked to estimate whether male or female names were more frequent on the list. The names of the famous celebrities were recalled more frequently compared to those of the less famous celebrities. The majority of the participants incorrectly judged that the gender associated with more famous names had been presented more often than the gender associated with less famous names. Tversky and Kahneman argue that although the availability heuristic is an effective strategy in many situations, when judging probability use of this heuristic can lead to predictable patterns of errors.

Schwarz and his colleagues, on the other hand, purposed the ease of retrieval explanation, in which is the ease with which examples come to mind, not the number of examples, is used to infer the frequency of a given class. In a study by Schwarz and colleagues to test their explanation, participants were asked to recall either six or twelve examples of their assertive or very unassertive behavior. Participants were later asked to rate their own assertiveness. Pretesting had indicated that although most participants were capable of generating twelve examples, this was a difficult task. The results indicated that participants rated themselves as more assertive after describing six examples of assertive compared with unassertive behavior condition, but rated themselves as less assertive after describing twelve examples of assertive compared with unassertive behavior condition. The study reflected that the extent to which recalled content impacted judgment was determined by the ease with which the content could be brought to mind (it was easier to recall 6 examples than 12), rather than the amount of content brought to mind.

Media
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 un-sensational 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.

Health
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.

Education
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. 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.

Criminal justice
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.

Perceived risk
Previous studies have indicated that explaining a hypothetical event makes the event seem more likely through the creation of causal connections. However, such effects could arise through the use of the availability heuristic; that is, subjective likelihood is increased by an event becoming easier to imagine.

Vividness effects
Two studies with 108 undergraduates investigated vivid information and its impact on social judgment and the availability heuristic and its role in mediating vividness effects. In Study 1, Subjects listened to a tape recording that described a woman who lived with her 7-yr-old son. Subjects then heard arguments about the woman's fitness as a parent and were asked to draw their own conclusions regarding her fitness or unfitness. Concrete and colorful language was found to influence judgments about the woman's fitness as a mother. Study 2 examined whether the presence vs absence of photographs would bias Subjects' estimates as to the percentage of Yale (vs Stanford) students in the sample of men and women whose names appeared on the original list and whether these percentages were causally related to the respondents' memory for the college affiliations of the individual students on the list. The presence of photographs affected judgments about the proportion of male and female students at the 2 universities. Such effects have typically been attributed to the ready accessibility of vividly presented information in memory—that is, to the availability heuristic.

Judging frequency and probability
In general, availability is correlated with ecological frequency, but it is also affected by other factors. Consequently, the reliance on the availability heuristic leads to systematic biases. Such biases are demonstrated in the judged frequency of classes of words, of combinatorial outcomes, and of repeated events. The phenomenon of illusory correlation is explained as an availability bias.

Ease of recall as a critique
One of the earliest and most powerful critiques of the original Tversky and Kahneman study on the availability heuristic was the Schwartz et al. study which found that the ease of recall was a key component in determining whether a concept became available. Many studies since this criticism of the original availability heuristic model have repeated this initial criticism, that the ease of recall factor became an integral facet of the availability heuristic itself (see Research section).

Alternative explanations
Much of the criticism against the availability heuristic has claimed that making use of the content that become available in our mind is not based on the ease of recall as suggested by Schwarz et al. For example it could be argued that recalling more words that begin with K than words with the third letter being K could arise from how we categorize and process words into our memory. If we categorize words by first letter, and recall them through the same process, this would show more support for the representativeness heuristic than the availability heuristic. Based on the possibility of explanations such as these, some researchers have claimed that the classic studies on the availability heuristic are too vague in that they fail to account for people's underlying mental processes. Indeed, a study conducted by Wanke et al. demonstrated this scenario can occur in situations used to test the availability heuristic. Future studies should be conducted to determine if and when this alternative explanation will occur.

A second line of study has shown that frequency estimation may not be the only strategy we use when making frequency judgments. A recent line of research has shown that our situational working memory can access long term memories, and this memory retrieval process includes the ability to determine more accurate probabilities. This finding suggests that more research should be conducted to determine how much memory activation affects the availability heuristic.