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Tversky and Kahneman (1973) proposed that when faced with the difficult task of judging probability or frequency, people use a limited number of strategies, called heuristics, to simplify these judgments. One such strategy is the availability heuristic, which refers to “the tendency to judge the frequency or likelihood of an event by the ease with which relevant instances come to mind” (Baumeister & Bushman, 2008). Life-long experience has shown us that instances of large classes are remembered better and quicker than instances of less common classes, that likely occurrences are easier to imagine than unlikely ones, and that associate connections are strengthened when two event repeatedly co-occur. Thus, a person may perhaps estimate the size of a class, the likelihood of an event, or the regularity of co-occurences by evaluating the ease with which the relevant mental operation of “retrieval, construction, or association” can be carried out (Tversky & Kahneman, 1973, p. 677). Nevertheless, overreliance on availability leads to predictable biases, some of which are illustrated below:
Biases due to the retrievability of instances: When the size of a class is judged by the availability of its instances, a class whose instances are effortlessly retrieved will seem more numerous than a class of equal frequency whose instances are not as easily retrievable (Tversky & Kahneman, 1974, p. 1127). Schwarz et al. (1991) tested this notion by having participants describe either very assertive or very unassertive behaviors in which they had engaged (students were informed that the study was concerned with developing role-playing scenarios that could be used in future relaxation training). Also, the participants were asked to describe either 6 or 12 examples of assertive, or unassertive, behavior. Subsequently, participants were later asked to rate their own assertiveness. The results showed that participants rated themselves as more assertive after describing 6, rather than 12, examples for the assertive behavior condition, and conversely rated themselves as less assertive after describing 6, rather than 12, examples for the unassertive behavior condition. The study reflected that the implications of recalled content were qualified by the ease with which the respective content could be brought to mind (easier to recall 6 examples, rather than 12) (Schwarz et al., 1991, p. 196).
Biases due to the effectiveness of a search set: In a well-known study by Tversky and Kahneman (1973, Experiment 3), 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?” The results showed that participants overestimated the number of words that began with the letter “k”, but underestimated the number of words that had “k” as the third letter. Tversky and Kahneman (1973) reasoned that people answer such a question by comparing the availability of the two categories by assessing the ease with which instances of the two categories come to mind. For most people, it is easier to think of words that begin with K, than words with K in the third position, so this is judged to be a more common occurrence. The psychologists reasoned that if the judgment of frequency is produced as a result of assessed availability, then words that start with K should be judged as more frequent. However, the truth is that a typical text contains twice as many words that have k as their third, rather than first, letter (Tversky & Kahneman, 1973, p.679).
Biases of imaginability: In some cases, people have to assess the frequency or probability of a class whose instances are not stored in memory but can be produced according to a specific rule. In such instances, people typically generate several instances and evaluate probability or frequency by the ease with which the relevant situations can be constructed. However, the ease of generating instances does not always mirror their actual frequency, and this mode of evaluation is prone to baises (Tversky & Kahneman, 1974). A hypothetical real-life instance that Tversky and Kahneman (1974) reference is the risk involved in an adventurous expedition. The psychologists suggest that the risk involved in an adventurous expedition, for example, is weighed by imagining contingencies with which the expedition is not suited to cope. If many of these difficult contingencies are heavily imagined, the expedition can be made to appear much more dangerous than actual facts suggest is likely. Contrastingly, the risk involved in the expedition may be largely underestimated if some possible dangers are either difficult imagine, or simply do not come to mind (1128).
Illusory correlation: Chapman and Chapman (1967) described a bias in the judgment of the frequency with which two events co-occur. Their demonstration showed that the co-occurrence of paired distinctive stimuli resulted in an overestimation of the frequency of such pairings (Chapman & Chapman, 1967). To test this idea, naïve judges were presented with information concerning several hypothetical mental patients. The data for each patient consisted of a clinical diagnosis and a drawing made by the patient. Later, the judges estimated the frequency with which each diagnosis (such as paranoia or suspiciousness) had been accompanied by various features of the drawing (such as peculiar eyes). The subjects markedly overestimated the frequency of co-occurrence of natural associates, such as suspiciousness and peculiar eyes. This effect was labeled illusory correlation. In their erroneous judgments of the data to which they had been exposed, naïve subjects “rediscovered” much of the common, but unfounded, clinical lore concerning the interpretation of the draw-a-person test (Chapman & Chapman, 1967). Tversky and Kahneman (1974) suggest that availability provides a natural account for the illusory-correlation effect. The strength of the associative bond between two events could provide the basis for one’s judgment of how frequently the two events co-occur. When the association is strong, one is likely to conclude that the events have been paired frequently. Consequently, strong associates will be evaluated as having occurred together frequently (1128).
Alternative explanations/critiques: In regards to imaginability errors creating biases in the availability heuristic, Levi and Pryor (1987) have suggested that perceived causes or reasons for the event, rather than imagery of the event itself, influence probability estimates. The psychologists provide evidence for this notion by pointing to a study of the 1987 presidential debate between Reagan and Mondale. Participants either simply imagined the winner of the debate, or imagined and considered reasons for why Reagan or Mondale would win the debate. Results demonstrated that simply imagining Reagan or Mondale winning the debate had no effect on predictions of who would win the debate. However, imagining/considering reasons for why Reagan or Mondale would win the debate did significantly affect predictions. (Levi & Pryor, 1987). Wanke, Schwarz, and Bless (1995) argue that although the availability heuristic has stimulated an enormous amount of research, the classic studies on the topic are surprisingly ambiguous regarding the underlying process (83). For example, in the aforementioned Tversky and Kahneman (1973, Experiment 3) study, Wanke et al. believe that seemingly this finding reflects that words that begin with a certain letter come to mind with greater ease than words that have a certain letter in the third position. However, this differential ease of recall, they suggest, may alter subjects’ frequency estimates in two different ways. In one scenario, as the availability heuristic suggests, the subjects may use the subjective experience of ease of difficulty of recall as a basis of judgment. Wanke et al. 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, Wanke et al. 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 overrepresented in the recalled sample, again producing a prediction of higher frequency. It must be noted, however, that in the second scenario the estimate would be based on recalled content rather than on the subjective experience of ease of recall (84).
Baumeister, Roy. Social Psychology & Human Nature Belmont, CA: Thomson Wadsworth, Inc., 2008. 161-162.
Chapman, L. J. (1967). Illusory Correlation in Observational Report. Journal of Verbal Learning and Verbal Behavior, 6, 151-155.
Levi, Ariel. & Pryor, J. (1987). Use of the Availability Heuristic in Probability Estimates of Future Events: The Effects of Imagining Outcomes versus Imagining Reasons. Organizational Behavior and Human Decision Processes, 40, 219-234.
Schwarz, N. et al. (1991). Ease of Retrieval as Information: Another Look at the Availability Heuristic. Journal of Personality and Social Psychology, 61 (2), 195-202.
Tversky, A. & Kahneman, D. (1973). Availability: A Heuristic for Judging Frequency and Probability. Cognitive Psychology, 5 (2), 677-695.
Tversky, A. & Kahneman, D. (1974). Judgments and Uncertainty: Heuristics and Biases. Science, New Series, 185 (4157), 1124-1131.
Wanke, M., Schwarz, N., & Bless, H. (1995). The Availability Heuristic Revisited: Experienced Ease of Retrieval in Mundane Frequency Estimates. Acta Psychologica, 89, 83-90.