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Permission from M. Dynan, L. H. Parker, and A. Ryan to use data from the evaluation of a new subject introduced into some secondary schools in Western Australia is gratefully acknowledged, as are discussions about the theoretical implications of the data with L. H. Parker. Alan Lyne wrote the computer programs used in the analysis of the data. Helpful comments on earlier versions of the chapter were provided by G. A. Douglas, 0. D. Duncan, G. N. Masters, and B. D. Wright.
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