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Diagnosticity

Definition

Diagnosticity refers to the extent to which a source of data can discriminate between a particular hypothesis and its alternatives. In social situations, individuals often observe others' behaviors and attempt to form an impression about their personality and aptitudes. As part of this process, individuals test dispositional hypotheses, namely, hypotheses regarding others' traits and abilities. Diagnostic sources of data are those that discriminate between possessing a particular trait or ability and not possessing the trait or ability.

For example, consider a situation in which a new classmate, John, does not respond to your welcome greetings and you want to know what kind of person he is. You may generate a hypothesis (e.g., “John is unfriendly”), gather information to test this hypothesis, and draw an inference based on the available information. Finding out that John “yelled at a fellow classmate in public” is highly diagnostic information, because such behavior is unlikely to occur unless John is an unfriendly person. Finding out that John dislikes parties has little diagnostic value. This information cannot distinguish between the hypothesis that John is unfriendly and the plausible alternative hypothesis that he is shy, as both unfriendliness and shyness may lead John to dislike parties.

Background

When testing a dispositional hypothesis, one of two broad strategies, distinguished by the extent to which individuals consider alternatives to their chosen hypothesis, may be undertaken. One strategy, called diagnostic hypothesis testing, is employed when individuals search for evidence that bears on both the plausibility of their focal hypothesis as well as on the plausibility of its alternatives. By this strategy, individuals gather information that can distinguish between their chosen hypothesis and alternative ones. Once sufficient information has been gathered, their confidence in their conclusion is high only if the evidence is largely consistent with the chosen hypothesis and inconsistent with its alternatives.

In contrast to diagnostic testing, a second strategy, called pseudodiagnostic hypothesis testing, involves gathering and using information only according to its consistency with the chosen hypothesis. Alternative hypotheses are ignored, or it is simply assumed that information that is consistent with the focal hypothesis is inconsistent with its alternatives. In the previous example, individuals would only ask about John's unfriendly behaviors and not about his shy behaviors. They would then draw their conclusion on the basis of the extent to which the evidence is consistent with being an unfriendly person, without considering if this evidence is also consistent with other possibilities such as being a shy person. Compared to diagnostic testing, pseudodiagnostic strategy is simple, fast, and relatively effortless. However, when evidence is both consistent with the focal hypothesis and its alternatives, pseudodiagnostic testing might lead to a confirmation bias, namely, a sense of confidence that the evidence supports one's chosen hypothesis when, in fact, alternative hypotheses may be true.

Evidence

Research by Yaacov Trope and his colleagues has demonstrated that individuals are sensitive to diagnosticity concerns when testing their hypotheses. That is, they consider alternative hypotheses when searching for information regarding a chosen hypothesis and weighing the evidence against these alternatives when drawing their inferences. For example, when individuals tested a hypothesis that a target person is an extravert, they preferred to ask questions about highly diagnostic introverted behaviors (being quiet) over questions about weakly diagnostic extraverted behaviors (engaging in athletic activities) and when testing a hypothesis that a target person is an introvert they preferred questions about highly diagnostic extraverted behaviors (being friendly) over weakly diagnostic introverted behaviors (listening to classical music). They were also more confident in their inferences when the answers to these questions provided more diagnostic evidence.

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