Statistical effects of varying sample sizes on the precision of percentileestimates

The present study evaluates the precision of outlying percentile estimates, with age- and sex-associated variations and facilitates decisions needed t o revise the current NCHS 1977 Growth Charts with regard to 1) the inclusio n of 3(rd) and 97(th) percentiles and 2) the selection of survey data for t he construction of the revised growth charts. Simulation was performed to o btain data with distribution characteristics similar to those of The Third National Health and Nutritional Examination Survey (NHANES III) (1988-1991) data. NHANES III consists of a two-phase, 6-year, complex stratified multi stage probability cluster, cross-sectional survey conducted from 1988 throu gh 1994 to represent the US noninstitutionalized population. Phase I of the survey consisted of 679 boys and 622 girls in age groups 3, 8, 13, and 18 years. Weight and stature, the body mass index (BMI) (weight/stature(2); kg /m(2)) was calculated. The results show that 1) the precision of the percen tile estimates is greater for stature than for weight and BMI, 2) percentil es during the pubertal period are less precise than those during the prepub ertal and postpubertal periods for weight and BMI but there is little diffe rence for stature, and 3) percentile estimates are more precise for girls t han boys for weight and BMI, but not for stature. The present findings sugg est that pooling of NHANES III and earlier National Center for Health Stati stics (NCHS) survey data is necessary to achieve reasonable precision for t he 3(rd) and 97(th) percentile estimates. (C) 2000 Wiley-Liss, Inc.