Measures of height and weight in the CHAS survey are based on the proxy reports and are not independently measured. Previous studies of height and weight estimates based on parent reporting compared to independent measurement have found that, for children under 10 years of age, height was generally underreported and weight was generally overreported (National Survey of Children's Health, 2003). CHAS has found a similar trend for proxy report of child height. A greater percentage of missing values and biologically improbable values (e.g., height reported too high for child's age) were found among younger age groups than among children ages 10 and older. Because height, and therefore BMI, is not consistently available for age, the data tables for BMI include only children ages 10 and older. For parents' report of child weight, the proportion of missing values and biologically improbable values were not found to be associated with child age. Data tables for Weight-for-Age, therefore, include children ages 2 and older.
Proxy reported measures of weight were collected for children 2 years old or older. Data tables presenting percentiles for Weight-for-Age include children ages 2 and older. The SAS code for calculating Weight-for-Age was obtained from the National Center for Health Statistic's Web site entitled, "2000 CDC Growth Charts: United States".
The calculations use the 2000 CDC growth charts, by age and sex, as the standard for assessing weight status. Calculation of Weight-for-Age is based on the age of the child in months. For those children where age was only available in years, children were assumed to be at the midpoint of the age-year for purposes of calculating Weight-for-Age. Outlier observations or observations that are considered to be "biologically implausible values" (BIVs; i.e., values identified as too low or too high for child age) based on World Health Organization fixed exclusion ranges were excluded from the CHAS data tables.
Percentiles are the most commonly used clinical indicator to assess the size and growth patterns of individual children in the United States. Percentiles rank the position of an individual by indicating what percent of the reference population the individual would equal or exceed. For example, a 5-year-old girl whose weight is at the 25th percentile weighs the same or more than 25 percent of the reference population of 5-year-old girls, and weighs less than 75 percent of the 5-year-old girls in the reference population. Weight-for-Age percentiles are used to measure a child's weight based strictly on age. It does not take into account the height of a child. This is not a method to determine obesity (or overweight) in children, but simply an indicator of growth as compared to children of the same age.
Body Mass Index-for-Age
Body Mass Index (BMI)-for-Age percentiles have emerged as the favored method to measure weight status in children. BMI is calculated in the same manner for children as it is for adults: weight (in kilograms) divided by height (in meters) squared. However, because children's body fatness changes over the years as they grow, and girls and boys differ in their body fatness as they mature, BMI for children is age- and sex-specific. The SAS code for calculating BMI-for-Age was obtained from the National Center for Health Statistic's website entitled, "2000 CDC Growth Charts: United States".
The calculations use the 2000 CDC growth charts, by age and sex, as the standard for assessing BMI status. Calculation of BMI-for-Age is based on the age of the child in months. For those children where age was only available in years, children were assumed to be at the midpoint of the age-year for purposes of calculating BMI-for-Age. Based on World Health Organization fixed exclusion ranges, outlier observations or observations that are considered to be "biologically implausible values" (BIVs; i.e. values identified as too low or too high for child age) were excluded from the CHAS data tables.
Categories for weight status are based on BMI-for-Age percentiles and defined as: less than 5 percent classified as underweight; between 5 and 84 percent classified as recommended range; between 85 and 94 percent classified as overweight; and greater than 95 percent classified as obese. For more information on assessment and calculation of BMI, please visit the CDC body mass index web page.