and requires an understanding of error and variation. As was the case for hypothesis generation and the design of experiments, the role of prior knowledge and beliefs has emerged as an important influence on how individuals evaluate evidence.

Covariation Evidence

A number of early studies on the development of evidence evaluation skills used knowledge-lean tasks that asked participants to evaluate existing data. These data were typically in the form of covariation evidence—that is, the frequency with which two events do or do not occur together. Evaluation of covariation evidence is potentially important in regard to scientific thinking because covariation is one potential cue that two events are causally related. Deanna Kuhn and her colleagues carried out pioneering work on children’s and adults’ evaluation of covariation evidence, with a focus on how participants coordinate their prior beliefs about the phenomenon with the data presented to them (see Box 5-1).

Results across a series of studies revealed continuous improvement of the skills involved in differentiating and coordinating theory and evidence, as well as bracketing prior belief while evaluating evidence, from middle childhood (grades 3 and 6) to adolescence (grade 9) to adulthood (Kuhn, Amsel, and O’Loughlin, 1988). These skills, however, did not appear to develop to an optimal level even among adults. Even adults had a tendency to meld theory and evidence into a single mental representation of “the way things are.”

Participants had a variety of strategies for keeping theory and evidence in alignment with one another when they were in fact discrepant. One tendency was to ignore, distort, or selectively attend to evidence that was inconsistent with a favored theory. For example, the protocol from one ninth grader demonstrated that upon repeated instances of covariation between type of breakfast roll and catching colds, he would not acknowledge this relationship: “They just taste different … the breakfast roll to me don’t cause so much colds because they have pretty much the same thing inside” (Kuhn, Amsel, and O’Loughlin, 1998, p. 73).

Another tendency was to adjust a theory to fit the evidence, a process that was most often outside an individual’s conscious awareness and control. For example, when asked to recall their original beliefs, participants would often report a theory consistent with the evidence that was presented, and not the theory as originally stated. Take the case of one ninth grader who did not believe that type of condiment (mustard versus ketchup) was causally related to catching colds. With each presentation of an instance of covariation evidence, he acknowledged the evidence and elaborated a theory based on the amount of ingredients or vitamins and the temperature of the

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