1. Body-weight, height, skinfold thickness and body density measurements were made on 245 men and 324 women aged between 17 and 72 years. The body fat content of each individual was calculated from his density measurement using equations similar to that of Siri (1956) but appropriate to age, muscular development, and extent of obesity. Regression equations were then derived for separate age-groups for the prediction of body fat from (1) weight–height (W–H) relationships (W:H, W:H2 W:H3, W0.33:H, H:W0.33), (2) ‘percentage overweight’ (body-weight:mean body-weight; W:W;) and ‘percentage desirable weight’ body-weight: ‘desirable weight’ (mean weights for men and women aged 20-24 years); (W:DES), (3) the independent variables weight and height incorporated in a regression equation, (4) skinfold measurements.
2. The correlations between height and indices, W:H, W:H3, W0.33:H and H:W0.33: were substantially different from those between height and body fat estimated by densitometry.
3. The method having the highest correlation with body fat estimated by densitometry was the skinfold method, although in the older groups of women other methods sometimes gave equally good correlations. The index H:W0.33 had a negative correlation with body fat, and the indices W:H, W0.33:H and W:H3 had a lower correlation with body fat estimated by densitometry than did the other indices. These indices are therefore unsuitable measures of obesity, and this is in agreement with the findings of other workers.
4. The correlations betweeen body fat estimated by densitometry and the indices W:H2, W:W, W:DES and the equations incorporating the independent variable weight and height are all very similar, although it is not appropriate to use the index W:W; as a measure of obesity in groups of people of widely different ages.
5. The standard error of prediction of body fat from skinfold measurement may be of the same order of magnitude as the standard error of prediction of body fat by densitometry. It is therefore probably inappropriate to assess the accuracy of the skinfold method by comparison with the density method alone.
6. From the distribution patterns obtained, it was evident that compared with the density method, all methods tended to over-estimate body fat in very lean individuals. The skinfold method however showed this tendency to only a relatively small extent.
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