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Resumo

Resistant delineation, a technique adapted from exploratory data analysis (Tukey, Exploratory Data Analysis, 1977), was applied to smooth age‐specific percentiles for triceps skinfold thickness across ages from 1 to 20 years. Row percentiles were transformed to logarithms to promote symmetry and to render variability more nearly homogeneous across ages. The delineation involved smoothing resistantly the sequences of age‐specific log medians and the sequence of age‐specific positive differences between the '4253H, twice' (Velleman, J. Am. Statist. Assoc., 75:609–615, 1980). The delineation concluded by recombining these smoothed sequences to obtain smoothed percentiles in the log scale. Finally, the logarithmic transformation was reversed, yielding the smoothed age‐specific percentiles. Comparisons of smoothed results from resistant, delineation with the original data indicated a satisfactory fit. Comparisons with published smoothed percentiles, obtained from the same data by a cubic‐spline procedure, showed that the resistant delineation captured the structure of the raw data better than the cubic‐spline procedure. The resistant delineation procedure makes few assumptions of the underlying data, it ensures a proper order relationship among the smoothed percentiles, it is relatively insensitive to isolated unusual data, and it is available in a common software package.

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