Notes for Data Users

2003 Data Limitations

The BRFSS is a cross-sectional surveillance survey currently involving 54 reporting areas (1). It is important to note that any survey will have natural variation across sample sites; therefore, some variation between states is to be expected. The complex sample design and the multiple reporting areas complicate the analysis of the BRFSS. Although CDC works with the states to minimize deviations, in 2003 there were some deviations in sampling and weighting protocols, sample size, response rates, and collection or processing procedures. In addition, California’s questionnaire had a few minor differences in wording of questions. The following section identifies other known variations for the 2003 data year.

A. 2003 Data Anomalies and Deviations from Sampling Frame and Weighting Protocols

In 75% of the states, a portion of sample records intended for use during one month took more than one month to complete. In several instances, states used their monthly sample over a period of several months. This deviation will disproportionately affect analyses based on monthly, rather than annual, data. Additionally, Michigan received its sample quarterly rather than monthly.

Several states did not collect data for all 12 months of the year. New Jersey did not report any interviews in July. The District of Columbia did not complete any interviews in May, June, July, and August. New Mexico did not complete any interviews in October and Ohio did not complete interviews in July and August.

Several states were unable to close out the December sample in 2003 and data collection continued into early 2004. Illinois, Kentucky, Nevada, New Mexico, Ohio, Oklahoma, Utah, and Wisconsin had some completed interviews in January, 2004. Hawaii completed some interviews in January and February, 2004.

More information about the quality of the survey data can be found in the 2003 BRFSS Summary Data Quality Report.

B. Other Limitations of the 2003 Data

Telephone coverage varies by state and also by subpopulation. Telephone coverage averages 97.6% for U.S. states as a whole, but noncoverage ranges from 1.1% in Connecticut and New Hampshire, to 6.6% in Mississippi. It is estimated that 23.8% of households in Puerto Rico are without telephone service. Data on telephone coverage in U.S. households are available at http://factfinder.census.gov.

Illinois used a dual questionnaire and collected data on core items addressing health status, health care access, exercise, diabetes, hypertension and cholesterol awareness, asthma, immunization, tobacco use, alcohol consumption, physical activity, and demographics from all eligible respondents. Questions about fruit and vegetable consumption, weight control, excess sun exposure, arthritis, falls, disability, veteran’s status, and HIV/AIDS were asked of about half of eligible respondents.

California modified the wording of core questions addressing health plans, diabetes, frequency of alcohol consumption, Hispanic ethnicity, level of education, and household income. The data from these questions may therefore have limited comparability to those of other reporting areas.

The data from an optional module is included if asked of all eligible respondents within a state for the entire data collection year. A state may have indicated the use of an optional module in 2003, but the data may have been moved into the state-added questions if it does not represent all eligible respondents.

A change in 2002 to the final disposition codes has continued to present some inconsistencies in closing out the questionnaire. Prior to 2002, interviews that were terminated during or after the demographics section were coded as complete interviews, and any remaining unanswered questions were coded as refused by the interviewer. In 2002, a revised procedure was implemented for handling partial completes. The revised procedure for partial completes is to stop coding questions at the point of interview termination to assign the appropriate disposition code. However, states have not consistently followed the procedure. During 2003, states generally handled partial complete interviews in one of three ways: they 1) coded the remaining questions as refused and coded the record a 110 Complete, 2) coded the remaining questions as refused and coded the record a 120 Partial Complete, or 3) did not ask the remaining questions (answers left as missing) and coded the record a 120 Partial Complete. The variability in how the interviews are dispositioned and where in the survey the interview was terminated will have an impact on refusal rates for certain questions and modules. These inconsistencies should be taken into account when determining which records to include in an analysis. Records with a termination in the questionnaire followed by coded refusals for the remainder of the eligible responses have been dispositioned as 120 Partial Completes.

Another issue regarding partial completes is the inappropriate coding of the remaining questions as “refused” (i.e., ‘9’) when some of these questions may have valid response codes of greater than ‘9.’ For example, some questions allow responses of 01-76, 77, 88, and 99 (with 99 as the refusal code). The problem occurs when an interviewer incorrectly codes the remaining questions as refused and enters a ‘9’ instead of a ‘99’ for these question response types. Nine (9) is a valid response for these particular questions and should not have been used to indicate refusal; doing so may have altered which questions were coded as refused for the remainder of a core section or module. When reviewing responses to a partial complete, data users should therefore be aware that a core section or module that follows the demographics section may contain questions incorrectly coded as refused (‘9 filled’).

Several states continue to ask the Diabetes module questions directly after the Diabetes questions in the core of the survey. In addition, several states ask the Adult Asthma module questions after the asthma questions in the core. Some states have also asked the Childhood Asthma module questions in the demographics section of the core survey after question 6, (CHILDREN) – number of children under age 18 in household.

More information about survey item non-response can be found in the 2003 BRFSS Summary Data Quality Report and in the respective states’ Data Quality Reports.

New Calculated Variables and Risk Factors

Not all of the variables that appear on the public use data set are taken directly from the state files. CDC prepares a set of SAS programs that are used for end of year processing. These programs prepare the data for analysis and add weighting, sample design, intermediate, calculated variables, and risk factors to the data set. The following calculated variables and risk factors, created for the user’s convenience, are examples of results from this procedure:

The procedures for the variables vary in complexity; some only combine codes, while others require sorting and combining selected codes from multiple variables, which may result in the calculation of an intermediate variable. For further details regarding the calculated variables and risk factors, refer to the document entitled “Calculated Variables and Risk Factors for the 2003 Behavioral Risk Factor Surveillance System,” available from the 2003 Technical Documents and Survey Data page.

The variables listed below are used in the BRFSS Prevalence Data and SMART BRFSS tables available on this Web site.

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This page last reviewed June 22, 2005

United States Department of Health and Human Services
Centers for Disease Control and Prevention
National Center for Chronic Disease Prevention and Health Promotion
Division of Adult and Community Health