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versión On-line ISSN 1668-7027


MORO, Rodrigo  y  BODANZA, Gustavo A. The debate around the facilitation effect on conditional probability problems: Is this a case of crucial experimentation?. Interdisciplinaria [online]. 2010, vol.27, n.1, pp. 163-174. ISSN 1668-7027.

In the early '70s, Tversky and Kahneman founded a research program in Cognitive Psychology called Heuristics and Biases. This program found extensive evidence that shows that people tend to commit reasoning errors when making judgments under uncertainty. A particular case is that people tend to fail when reasoning about conditional probability problems, that is, problems that ask for the probability of some event given the fact that another event has occurred (e.g. the probability of raining given that it is cloudy). But in the mid '90s, Gigerenzer and other evolutionary psychologists came along and gave an important turn to the state of the art. They showed that if the conditional probability problems used in the literature are framed in a different way, people's performance greatly improves. More specifically, if the problems present the information under a specific format called natural frequency format, around 50% of participants get the correct answer. Since the mid ´90s researchers engage in an important debate on how to account for such a facilitation effect. There are two main proposals, one by the Evolutionary Psychology Program and the other by Heuristic and Biases Program. The natural frequency hypothesis supported by the Evolutionary Program basically says that the natural frequency format is the responsible factor for the improvement in people's performance. The Heuristic and Biases Program, in turn, has proposed the nested-set hypothesis to explain the facilitation effect. The basic idea is that natural frequency versions tend to make transparent the relevant subset relations of the problem. When people see clearly the set relations involved in this kind of problems (the argument goes) they tend to use correctly base rates and thus, their performance improves. They point out that, according to this view, the success of the frequency effect does not have to do with natural frequency formats per se. They predict that any format whatsoever that make the relevant set relations clear will show the same effect. The key question is, then, as follows. Is this a case of crucial experimentation in favor of one of our rival hypotheses? In other words, is there an experiment or a series of experiments such that our rival hypotheses predict opposite results, so that we can claim one of them as victorious over the other? The empirical evidence on the matter is mixed. Some studies seem to support the natural frequency hypothesis while others seem to support the nested-set hypothesis. We will then try to clarify this debate by focusing on the diverse strategies and techniques used in the literature to settle the dispute. We will argue that the right strategy to discriminate between both hypotheses is to use genuine probability problems with a clarified set structure and see whether these conditions elicit or not a performance comparable to the natural frequency effect. Within this general strategy, we review the literature and found that there are three techniques, namely, the improved wording technique, the natural chance technique and the graphical representation technique that seem to provoke a performance as good as the one elicited by natural formats, giving, thus, a stronger support for the nested set hypothesis. However, a careful analysis shows that neither the improved wording technique nor the chance technique has provided both consistent and clear results in favor of the nested-set hypothesis. As for the graphical representation technique, the evidence still seems very slim. The improvement in performance was shown in two studies that worked with only one problem each. Furthermore, neither of these problems seems completely adequate. Thus, we do not think the last word about the matter has been said and more empirical work is needed to settle the issue.

Palabras clave : Cognitive psychology; Conditional probability; Facilitation effect; Heuristic and biases program; Evolutionary program; Natural frequency hypothesis; Nested-set hypothesis.

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