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Resistant cross‐ age smoothing of age‐specific percentiles for growth reference data
Author(s) -
Himes John H.,
Hoaglin David C.
Publication year - 1989
Publication title -
american journal of human biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.559
H-Index - 81
eISSN - 1520-6300
pISSN - 1042-0533
DOI - 10.1002/ajhb.1310010205
Subject(s) - percentile , smoothing , mathematics , homogeneous , statistics , smoothing spline , logarithm , raw data , algorithm , computer science , combinatorics , mathematical analysis , bilinear interpolation , spline interpolation
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.