The four-card selection task Wason, 1966 is one of the best known tasks used in the literatures of human reasoning. This article analyzes this selection task by using item response theory Lord & Novick, 1968 . Japanese undergraduates N=323 responded six types of the Wason's selection tasks including indicative task Wason, 1966 , beer task Griggs & Cox, 1982 , and cassava task Cosmides, 1989 . An exploratory categorical factor analysis revealed a one factor structure of the six tasks. The

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... of an analysis using a two-parameter logistic model indicated that the indicative tasks were similar to the beer task and the cassava task in terms of the discrimination parameter, and that the relative di阿culty between the tasks would varied according to the value of the ability parameter estimated by the two-parameter logistic model.