Georgetown University Medical Center

Department of Pediatrics

Washington DC 20007

The original data were published by the NCHS as cubic spline-smoothed percentile curves (1). Each percentile was actually a composite of several segments, since a single spline would not describe the entire curve well. Length as used in this program is a combination of recumbent length for subjects less than 24 months and stature for older subjects.

The height and weight curves were not normally distributed, making comparisons at different ages difficult. Dibley et al (5) derived normalized curves from the original curves and published the coefficients for these curve. The coefficients for normalized head circumference curves are available at the NetScut site. Using these normalized curves, it is possible to calculate Z-scores (multiples of the standard deviation) for any given observation. These scores are easily comparable at different ages, and are more useful as the extremes of measurement, as might be found in areas where malnutrition is present (6,7). All other statistical parameters are based on the Z-scores.

For each observation, the median (50th percentile) value is calculated. For instance, for a male subject of 15 months age, the median height is about 50 cm. If the observation is greater than the 50th percentile, a Z-score is generated based on a the relative distance between the Z-scores for the 50th and 97th percentiles. If the observation were less than the 50th percentile, the Z-score would be based on the 50th and 3rd percentile. Both 97th and 3rd percentiles are about 1.88 standard deviations from the median, and the distances from median are computed proportional to this value. In deriving the 3rd and 97th percentile curves from the original data, the 97th was based on the averaged standard deviations of the 75th, 90th and 95th percentiles, while the 3rd percentile curve was based on the 5th, 10th and 25th percentiles to help counter the skew present in the original curves.

If a Z-score cannot be calculated for a given observation, the program is unable to calculate the derived values, but will attempt to determine the value for which the observation is the median. For example, if a male subject has a head circumference of 48.5 cm, but is four years old, the Z-score cannot be calculated because it is undefined above 36 months of age. However, the program can determine that the 48.5 cm is the median value for a subject of about 18 months of age, hence this subject has a small head for age.

Several other values are based on the Z-score. These include the percentile, the absolute value relative to the median, and the percent of median. The percentile is calculated using an algorithm developed by Hill (8). The weight and length curves are defined only up to about 18 years, and head circumference curves are defined up to 36 months. Above these ages, the script will generate error messages. The weight-for-length curves are defined between a lower limit of 49 centimeters and a gender-specific upper limit of about 140 cm. For very large or small observed values may not correspond to any median value within these ranges, and an error message will be generated.

The program is designed so that errors will be trapped. Any value which cannot be calculated is represented in the output as an asterisk, and an error message appears below the entry.

- National Center for Health Statistics. NCHS Growth Curves for Children 0-18 Years, United States. Vital and Health Statistics. Health Resources Administration. Washington, D.C.: United States. Government Printing Office. 1977; Series 11-no. 165.
- Hammill PV, Drizd TA, Johnson CL, Reed RB, Roche AF, Moore WM. 1979. Physical growth: National Center for Health Statistics percentiles. Am J Clin Nutr 1979;32:607-626.
- WHO working group. Use and interpretation of anthropometric indicators of nutritional status. Bull WHO 1986;64:929-41.
- Gorstein J. Assessment of nutritional status: effects of different methods to determine age on the classification of undernutrition. Bull WHO 1989;67:143-50.
- Dibley MJ, Goldsby JB, Staehling NW, Trowbridge FL. Development of normalized curves for the international growth reference: historical and technical considerations. Am J Clin Nutr 1987;46:736-48.
- Dibley MJ, Staehling N, Nieburg P, Trowbridge FL. Interpretation of Z-score anthropometric indicators derived from the international growth reference. Am J Clin Nutr 1987;46:749-62.
- Waterlow JC, Buzina R, Keller W, Lane JM, Nichaman, MZ, Tanner JM. The presentation and use of height and weight data for comparing the nutritional status of groups of children under the age of 10 years. Bull WHO 1977;55:489-498.
- Hill ID. Algorithm AS 66. 1973;Appl Stat 22:424.
- Sullivan K, Gorstein J, Coulombie D. Epi Info Online Manual. Chapter 23, Example: Programs for Nutritional Anthropometry. URL = "http://www.cdc.gov/epo/epi/intro/manual/manchp23.htm"
- Backstrand, AR. Z-scores on the Internet. 1996;URL="http://www.nyu.edu/education/nutrition/software/zscore.htm"
- Bender B, Remancus S. Anthropometry Resource Center. 1999; URL="http://www.odc.com/anthro/"

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Last modification: June 24, 1999