Methodology for Behavioral Risk Factor Surveillance System for Obesity, Physical Activity and Fruit and Vegetable Consumption Rates
Rates and Rankings Methodology1
The analysis in The State of Obesity compares data from the Behavioral Risk Factor Surveillance System.
BRFSS is the largest ongoing telephone health survey in the world. It is a state-based system of health surveys established by CDC in 1984. BRFSS completes more than 400,000 adult interviews each year. For most states, BRFSS is the only source of population-based health behavior data about chronic disease prevalence and behavioral risk factors.
BRFSS surveys a sample of adults in each state to get information on health risks and behaviors, health practices for preventing disease and healthcare access mostly linked to chronic disease and injury. The sample is representative of the population of each state.
Washington, D.C., is included in the rankings because CDC provides funds to the city to conduct a survey in an equivalent way to the states.
The data are based on telephone surveys by state health departments, with assistance from the CDC. Surveys ask people to report their weight and height, which is used to calculate BMI. Experts say rates of overweight and obesity are probably slightly higher than shown by the data because people tend to underreport their weight and exaggerate their height.2
BRFSS made two changes in methodology for its dataset starting in 2011 to make the data more representative of the total population. The changes included making survey calls to cell phone numbers and adopting a new weighting method:
- The first change is including and then growing the number of interview calls made to cell phone numbers. Estimates today are that three in 10 U.S. households have only cell phones.
- The second is a statistical measurement change, which involves the way the data are weighted to better match the demographics of the population in the state.
The new methodology means the BRFSS data will better represent lower-income and racial and ethnic minorities, as well as populations with lower levels of formal education. Although generalizing is difficult because of these variables, it is likely that the changes in methods will result in somewhat higher estimates for the occurrence of behaviors that are more common among younger adults and to certain racial and ethnic groups.
The change in methodology makes direct comparisons to data collected prior to 2011 difficult.
Definitions of Obesity and Overweight
Obesity is defined as an excessively high amount of body fat or adipose tissue in relation to lean body mass.3,4 Overweight refers to increased body weight in relation to height, which is then compared to a standard of acceptable weight.5 Body mass index is a common measure expressing the relationship (or ratio) of weight to height. The equation is:
Adults with a BMI of 25 to 29.9 are considered overweight, while individuals with a BMI of 30 or more are considered obese.
For children, overweight is defined as a BMI at or above the 85th percentile and lower than the 95th percentile for children of the same age and sex; childhood obesity is defined as a BMI at or above the 95th percentile for children of the same age and sex; and severe childhood obesity is defined as a BMI greater than 120 percent of 95th percentile for children of the same age and sex.
BMI is considered an important measure for understanding population trends. For individuals, it is one of many factors that should be considered in evaluating healthy weight, along with waist size, body fat composition, waist-to-hip ratio, blood pressure, cholesterol level and blood sugar.6
Methodology for Obesity and Other Rates Using BRFSS
Data for this analysis was obtained from the Behavioral Risk Factor Surveillance System dataset (publicly available on the web at www.cdc.gov/brfss). The data were reviewed and analyzed for TFAH and RWJF by Daniel Eisenberg, PhD, Associate Professor, Health Management and Policy at the University of Michigan School of Public Health.
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. BMI was calculated by dividing self-reported weight in kilograms by the square of self-reported height in meters. The variable 'obesity' is the percentage of all adults in a given state who were classified as obese (where obesity is defined as BMI greater than or equal to 30). Researchers also provide results broken down by race/ethnicity — researchers report results for Whites, Blacks and Latinos — and gender. Another variable, 'overweight' was created to capture the percentage of adults in a given state who were either overweight or obese. An overweight adult was defined as one with a BMI greater than or equal to 25 but less than 30. For the physical inactivity variable a binary indicator equal to one was created for adults who reported not engaging in physical activity or exercise during the previous thirty days other than their regular job. For diabetes, researchers created a binary variable equal to one if the respondent reported ever being told by a doctor that he/she had diabetes. Researchers excluded all cases of gestational and borderline diabetes as well as all cases where the individual was either unsure, or refused to answer.
To calculate prevalence rates for hypertension, researchers created a dummy variable equal to one if the respondent answered "Yes" to the following question: "Have you ever been told by a doctor, nurse or other health professional that you have high blood pressure?" This definition excludes respondents classified as borderline hypertensive and women who reported being diagnosed with hypertension while pregnant.
1 Description of BRFSS and changes in methodology provided by CDC.
2 Merrill RM and Richardson JS. Validity of Self-reported Height, Weight, and Body Mass Index: Findings from the National Health and Nutrition Examination Survey, 2001-2006. Preventing Chronic Disease, 6(4):2009 (accessed March 2010).
3 Stunkard AJ and Wadden TA. Obesity: Theory and Therapy. Second ed. New York, NY: Raven Press, 1993.
4 National Research Council. Diet and Health: Implications for Reducing Chronic Disease Risk. Washington, D.C.: National Academy Press, 1989.
6 Parker-Pope T. "Watch Your Girth." The New York Times May 13, 2008.