Persuasive Communication - How Audiences Decide. 2nd Edition

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Heuristics and Biases in Audience Decision Making 223

exposure to bodily harm. Attempts to promote healthy behaviors by providing consumers with


probabilistic information are generally unsuccessful.^156


Sample-Size Insensitivity: The Intuitive Appeal of Examples


A second bias related to the representativeness heuristic is insensitivity to sample size. Although


sample size is fundamental to statistics, consideration of sample size is rarely a part of the audience’s


intuitive decision-making process.^157 Advertisers’ statements such as “Four out of fi ve dentists


surveyed recommend sugarless gum for their patients who chew gum” persuade many consumers


despite the fact that the survey results are statistically meaningless without mention of the number


of dentists surveyed. Even when audiences receive information about a sample’s typicality, they will


often fail to use it.^158


Instead of basing their decisions on the appropriately sized sample, audiences tend to overgen-

eralize from a single, easy-to-comprehend but unrepresentative example.^159 TV viewers often base


their perceptions of newsworthy events on a single example provided to them by the media.^160 The


person or event featured in the news story supposedly exemplifi es a whole category. Even when


viewers are presented with accurate information, frequent exposure to such exemplars can lead


them to make biased decisions.^161


A single example can have a stronger impact on audience decision making than statistics.^162 For

example, mock jurors in a simulated capital sentencing hearing were more persuaded by a psy-


chologist who gave his personal clinical assessment of the dangerousness of the defendant than by a


psychologist who presented a statistically accurate actuarial assessment.^163 In a study that presented


the same information in a statistical and nonstatistical format, one group of high school teachers


read information about a new science curriculum that was in the form of a case study written by


a single teacher who had used the curriculum. The same information was given to another group


but in the form of a statistical summary of the fi ndings of 12 teachers who had used the curricu-


lum. The high school teachers found the case study to be much more persuasive than the statistical


summary.^164


Audiences are especially likely to disregard sample size when they encounter extreme exam-

ples.^165 Audiences are also likely to disregard sample size when they encounter well-known


examples. For example, audiences weight identifi able victims more heavily than statistical vic-


tims.^166 As behavioral economists George Loewenstein and Jane Mather have observed, when


Rock Hudson and Magic Johnson were diagnosed with AIDS, the public’s concern for the


disease skyrocketed.^167


Surprisingly, experts, even expert statisticians, regularly fail to take sample size into account.^168

A study of senior auditors planning an audit of a company’s internal control procedures fi nds that


auditors do not use the optimal sample size even when given all of the necessary information. Nei-


ther do they use the sample-size selection methodology the American Institute of Certifi ed Public


Accountants (AICPA) prescribes, nor do they notice sample-size errors.^169


The Causality Bias: The Appeal of Narratives and Stories


Audiences give the same information more weight when it is presented to them in chronological


order as an easy-to-comprehend narrative or story than when it is presented as a hard-to-comprehend


random list of facts. In a study comparing two versions of a travel brochure, one group of consum-


ers read a version of the brochure that described a vacation using a narrative form. Another group


read a version that described the vacation in a list form. Consumers who read the narrative version


evaluated the vacation much more positively than those who read the list of facts.^170

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