APPLICATION OF BODY MASS INDEX PRINCIPLES: METHODS

16 Dec
2009

After receiving parental permission, 66 children from grades one through five in the City of Richmond’s first model elementary school were assessed for height and weight (counterweight scale) by the school nurse on March 14, 2003. Students wore their school dress, took off their shoes, and emptied their pockets before being weighed. Additional demographic information obtained included grade in school, birth date, sex, and race. These 66 students were randomly selected from an overall study body of 283 students (first grade: 43, second grade: 50, third grade: 65, fourth grade: 64, and fifth grade: 60 students).


Table 1. Percentiles Linked to Z-Scores Between 0 and 3.5 in Increments of 0.5 If a student has a BMI z-score of 2.00, that student’s BMI is greater than 97.7% of the population described in the 2000 CDC Growth Charts.

Z-ScorePercentile

0.00

50.00

0.50

69.15

1.00

84.13

1.50

93.32

2.00

97.72

2.50

99.38

3.00

99.87

3.50

99.98

The Nutstat module of Epi Info allows the user to select normative data from 1978 CDC and 2000 CDC figures. In this study, we used the 2000 CDC data. Information entered included birth date, date of measurement, sex, height, and weight. Outcome information included age in months, BMI, percentile, and z-score. Age in months is self explanatory.
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Figure 1. CDC Growth Charts for BMI for Age

Figure 1. CDC Growth Charts for BMI for Age in Percentiles for Boys Ages 2-20 Years’

BMI is calculated as weight in kg divided by height (stature) in meters squared or as pounds divided by height in inches squared and this value multiplied by 703 to convert to kg/m2. In the Nutstat module, the clinician has the option of entering height and weight using either system of measurement. In this study, we entered height in inches and weight in pounds. cialis canadian pharmacy

Figure 2. CDC Growth Charts for BMI for Age

Figure 2. CDC Growth Charts for BMI for Age in Percentiles for Girls Ages 2-20 Years

Percentile (or percentile rank) reflects BMI rank compared with peers of the same sex and age. Figures 1 and 2 are CDC Growth Charts for BMI for age in percentiles for boy and girls, respectively, ages 2-20 years. A subject at the 95th percentile in BMI for sex and age has a BMI in the upper 5% of BMI measurements. That is, these are the children that weigh the most for their height. erectalis 20

Figure 3. Area to the Left of a Z-Score

Figure 3. Area to the Left of a Z-Score in a Normal (Gaussian) Distribution

In a normal (gaussian) distribution, the z-score represents the number of standard deviations (SD) away from the population mean. In other words, it indicates the degree to which an individual’s measurement deviates from what is expected for that individual. Z-scores and percentiles are inextricably linked in the normal (gaussian) distribution, the CDC Growth Charts, and the Nutstat module of Epi Info. Figure 3 depicts the area to the left of a z-score in a normal (gaussian) distribution. Table 1 links z-scores between 0 and 3.5 in increments of 0.5 to percentiles. Z-scores, because of their normal distribution, may appropriately undergo parametric statistical analysis. Z-scores are especially useful for comparing performance on several measures, each with a different mean and SD.

STATISTICS

T-statistics were used to compare z-scores.

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