The Behavioral Risk Factor Surveillance System (BRFSS)
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What is the BRFSS?
The BRFSS, administered by individual states and supported by the Division of Adult and Community Health, National Center for Chronic Disease Prevention and Health Promotion, CDC, is an on-going data collection program designed to measure behavioral risk factors in the U.S. adult, noninstitutionalized, civilian population. The objective of the BRFSS is to collect uniform, state-specific data on preventive health practices and risk behaviors that are linked to chronic diseases, injuries, and preventable infectious diseases. Factors assessed by the BRFSS include safety-belt use, tobacco use, physical activity, diet, and use of cancer screening services, among others. Data are collected from a random sample of adults (one per household) through a telephone survey.
What is its use?
Health departments use the data for a variety of purposes. Among those are to identify demographic variations in health-related behaviors, target services, address emergent and critical health issues, propose legislation for health initiatives and measure progress toward achieving state and national health objectives.
Who is covered in the BRFSS?
The health characteristics estimated from the BRFSS pertain only to the adult, civilian, noninstitutionalized population age 18 years and older. Respondents are identified through telephone-based methods. Although 95 percent of U.S. households have telephones, coverage ranges from 87-98 percent across states and varies for subgroups as well. No direct method of compensating for non-telephone coverage is employed by the BRFSS; however, post-stratification weights are used, and may partially correct for any bias caused by non-telephone coverage. These weights adjust for differences in probability of selection and nonresponse, as well as noncoverage, and must be used for deriving representative population-based estimates of risk factor prevalences.
Why are BRFSS results reported using 'weighted estimates'?
Unweighted data on the BRFSS are the actual responses of each participant. Unweighted data represent results before any adjustment is made for variation in respondents' probability of selection, for disproportionate selection of population subgroups relative to the state's population distribution, or nonresponse. Weighted BRFSS data represent results that have been adjusted to compensate for such differences.
Can I use a standard statistical package to estimate rates from the BRFSS?
Yes and no. You can weight your analysis and get valid point estimates (percentage points, means, etc.). However, it is not possible to accurately estimate the point estimates' confidence interval. The procedures for estimating variance given in most statistical texts and the programs available in most statistical software packages are based on the assumption of simple random sampling (SRS). The data collected in the BRFSS are obtained through a complex sample design, and therefore the direct application of standard statistical methods for variance estimation and hypothesis testing may yield misleading results. There are computer programs available which take such complex sample designs into account. SUDAAN and EpiInfo's C-Sample are among those suitable for analyzing BRFSS data. EpiInfo's C-Sample can be used to calculate simple frequencies and two-way cross-tabulations. SUDAAN can be used for both descriptive and inferential statistical methods.
What is the rule of 50?
Even though the overall number of persons in this survey is quite large for statistical inference purposes, subgroup analysis can lead to estimators that are unreliable. Consequently, analysis of subgroups, especially within a single data year or geographic area, requires that the user pay particular attention to the subgroup sample size. Small sample sizes may produce unstable estimates. Reliability of an estimate depends on the actual unweighted number of respondents in a category, not on the weighted number. Interpreting and reporting weighted numbers that are based on a small, unweighted number of respondents can mislead the reader into believing that a given finding is much more precise than it actually is. The BRFSS follows a rule of not reporting or interpreting percentages from questions whose unweighted number of valid responses is less than 50.
Can I combine years of data to get counts of 50?
When data from one time period are insufficient for estimating the prevalence of a risk factor, data may be combined for several periods as long as the periods being combined are not times during which the prevalence of the risk factor of interest has been substantially changing. When the prevalence of risk factors does not change rapidly over time, data combined for two or more years may provide a sufficient number of respondents so that additional prevalence estimates can be made for population groups (such as age/sex/race subgroups or county populations). This procedure requires that the weights applied to the sample be adjusted.
What can I do when there is not enough data for my county?
Sample sizes may still be inadequate for risk factor estimates for some geographic areas (e.g., counties) or subpopulations (e.g., persons over 85 years of age) even after combining data for several years. In such situations, the analyst may wish to derive synthetic estimates by extrapolating from the BRFSS data collected at the state level. Synthetic estimates can be calculated by using the population estimates for the subgroup of interest and the state BRFSS risk factor prevalences for that subgroup.