Methodology and Data Sources

What is Obesity? 

Having a body weight that is higher than what is considered as healthy for a given height is described as overweight or obesity. There are many methods of measuring body fat, some of which are expensive and time consuming. Body mass index, which is inexpensive and easy to calculate, is typically used as a proxy. Health officials recommend that individual health assessments should consider other factors as well. Research has demonstrated that a high BMI is strongly correlated with the same negative health consequences as high body fat, although the association between BMI does vary among ethnic groups.

BMI is a person’s weight in kilograms divided by his or her height in meters squared. For measurements in pounds and inches, BMI is calculated using the following formula:

Defining Obesity Among Children and Teens
Because kids are still growing, obesity is measured differently among children than adults. Instead of a simple BMI measurement, a child’s BMI is compared to others of the same age and sex.

According to the Centers for Disease Control and Prevention, child obesity is defined as a BMI that is at or above the 95th percentile for children and teens of the same age and sex. Overweight is defined as a BMI that is at or above the 85th percentile and below the 95th percentile for children and teens of the same age and sex.

Why is BMI age- and sex-specific for children and teens?
A child’s weight status is determined using an age- and sex-specific percentile for BMI, which is different from BMI categories used for adults. Because children’s body fat levels change over the course of childhood and vary between boys and girls, their BMI levels are expressed relative to other
children of the same age and sex.

BMI Levels and Weight Classifications

Obesity Rates and Trends Data

This site relies on multiple survey instruments to paint a complete picture of childhood obesity in America:

The National Health and Nutrition Examination Survey (NHANES) at CDC measures obesity rates among Americans ages two and older and is the primary source for national obesity data in this report of children ages 2 to 19. NHANES is particularly valuable in that it combines interviews with physical examinations and covers a wide age range of Americans. However, due to the delay between collection and reporting, the timeliness of its data can lag. 

The WIC Participant and Program Characteristics (WIC PC), a USDA survey analyzed for obesity trends by CDC, is a report of the Special Supplemental Nutrition Program for Women, Infants, and Children on the results of its biennial census of families served by the program. The data it collects include height and weight information. Because the program provides assistance only to low-income mothers and children under the age of 5, this dataset is limited. However, because obesity disproportionately affects the poor, and early childhood is a critical time for obesity prevention, the dataset provides valuable information for evaluating the effectiveness of programs aimed at reducing obesity rates and health disparities.

The National Survey of Children’s Health (NSCH) surveys parents of children ages 10 to 17 about all aspects of their children’s health, including height and weight. An advantage of this survey is that it includes both national and state-by-state data, so obesity rates between states can be compared. A disadvantage is that it relies on parent reports, not direct measures. Starting in 2016, the survey is conducted annually, but because the methodology changed in 2016, it is not possible to compare data collected previously with data collected in 2016 or later. Trends can be evaluated starting in 2016 and moving forward. 

The Youth Risk Behavior Surveillance System (YRBSS) tracks high-risk health behaviors among students in grades 9-12, including dietary behaviors and physical inactivity. The survey also measures the prevalence of obesity by asking respondents about their height and weight. As in other surveys that use self-reported data to measure obesity rates, this survey likely underreports the true rates. The survey is conducted in odd-numbered years. The most recent public YRBSS obesity data are from the 2017 survey.

Because research shows that children who have obesity at an early age are more likely to have obesity later in life, this site also includes the latest findings from the Behavioral Risk Factor Surveillance System (BRFSS), which tracks state-by-state adult obesity rates. BRFSS is an annual cross-sectional survey designed to measure behavioral risk factors in the adult population (18 years of age or older) living in households. Data are collected from a random sample of adults (one per household) through a telephone survey. The BRFSS currently includes data from 50 states, the District of Columbia, Puerto Rico, Guam and the Virgin Islands. Variables of interest included BMI, physical inactivity, diabetes, hypertension and consumption of fruits and vegetables five or more times a day.