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A factor analysis was conducted of a test battery designed for use with Belgian secondary school students and based on Thurstone's earlier work on the primary mental abilities. The analysis provided construct validity evidence for the existence of verbal, numerical, spatial, and reasoning abilities in the Belgian sample and thus contirmed the theoretical conceptions guiding the development of the test battery. An examination of the factorial organization of these mental abilities for different age groups revealed a progressive differentiation of abilities with age. There was also evidence that intensive academic specialization improves performance in the abilities required by these specializations.

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