Effect of like-sexed siblings on crown dimensions.

Normative values for mesiodistal and buccolingual crown dimensions are often derived from study samples containing siblings (MOORREES et al, J Dent Res 36:39-47, 1957; GARN, LEWIS, and WALENGA, Arch Oral Biol 13:841-844, 1968). In such instances the genetic N is obviously smaller than the number of subjects, and measures of central tendency and dispersion may both be affected by the "loading" of dimensionally-extreme sibships. The effects of such loading have not been ascertained, however, so that the propriety of using sibling-containing samples is as yet unknown. To resolve this question we have made use of mesiodistal (and buccolingual) crown-size dimensions of 208 subjects (109 boys and 99 girls) based on measurement of an average of 3 independent casts per individual (MOYERS et al, Standards of Human Occlusal Development, 1976), with values as given in the table. In addition, we calculated means and standard deviations on a subsample excluding 29 likesexed sibling pairs (11 sets of 2, 4 sets of 3 and