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A relatively small number of studies of the Stroop task has examined individual differences in age, sex, hemispheric processing, and language. The amount of interference is the primary dependent measure in most studies, not the factors that contribute to the interference. In the present target article, cluster analysis is used to identify groups of participants who respond similarly on the Stroop task. Integrated color-word Stroop stimuli were presented for varying durations in the first study. Significant individual differences were found. A cluster analysis identified two groups of subjects. One group responded consistently across durations and conditions while the other responded more erratically. Potential sources of individual differences were examined in a second study. 120 subjects were given the Color and Word Test along with selected subtests of the Stanford Binet Intelligence Test, age appropriate Wechsler tests, and the Detroit Tests of Learning Aptitude. Again, cluster analysis found two groups of subjects. The group with higher scores on visual reasoning and short-term memory produced more interference.

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