A common finding in confidence research is the hard-easy effect, in which judges exhibit greater overconfidence for more difficult sets of questions. Many explanations have been advanced for the hard-easy effect, including systematic cognitive mechanisms, experimenter bias, random error, and statistical artifact. In this article, I mathematically derive necessary and sufficient conditions for observing a hard-easy effect, and I relate these conditions to previous explanations for the effect. I conclude that all types of judges exhibit the hard-easy effect in almost all realistic situations. Thus, the effect's presence cannot be used to distinguish between judges or to draw support for specific models of confidence elicitation.

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