ABSTRACT This paper presents the results of a study designed to investigate the pseudodiagnosticity bias as a failure to identify and select diagnostically relevant information. The reported experiment (N = 240) aims to deepen understanding of the role played by the rarity of evidential features in a classical pseudodiagnosticity task. The problem used for the experiment was a classical pseudodiagnosticity task. Six experimental versions were constructed: they differed in the rarity of features proposed and in the percentages (high or low) associated with them. The results show that people’s responses appear to be influenced by the percentage values associated with explicit information more than by a rarity factor. When an initial piece of evidence is associated with a low percentage, the percentage of normatively diagnostic answers is greater than when this percentage is high. Furthermore, rarity is not, in itself, a crucial factor in the occurrence of pseudodiagnosticity bias. Rather, the perception of the difference between two evidential features in terms of informative value influences people’s responses when orienting a diagnostic evaluation. When people perceive an initial piece of evidence as having greater informative value than a second piece of evidence, they tend to (correctly) move their attention from the focal hypothesis to the alternative one.
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