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Lester E. Harrell, Jr., A Comparison of the Development of Oral and Written Language in School-Age Children, Monographs of the Society for Research in Child Development, Vol. 22, No. 3, A Comparison of the Development of Oral and Written Language in School-Age Children (1957), pp. 1-77
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Lester E. Harrell (1957) studied this question.
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