Given the growing popularity of cephalometric programs for the personal computer, it is once again necessary for the specialty to confront the problem of prediction accuracy. The strength of the relationships upon which a prediction scheme is based is often assessed by means of the coefficient of linear correlation, r. Although it is common to judge the practical significance of a relationship by squaring the correlation coefficient, the present paper argues that the index of forecasting efficiency, the percentage reduction in error, is not only the more appropriate index, but also one that is easy to infer directly from r.

Supported by N.I.D.R. grant DE08716; this help is greatly appreciated. In addition, the author wishes to thank Drs. Carroll-Ann Trotman, Fedon Livieratos, and James McNamara for their constructive comments during the preparation of this manuscript.

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Author notes

L.E. Johnston, Jr. is the Robert W. Browne Professor of Dentistry and Chairman, Department of Orthodontics and Pediatric Dentistry, The University of Michigan, Ann Arbor, MI