Behavioral Risk Factor Survey Module Technical Notes

Wisconsin Behavioral Risk Factor Survey (BRFS)

The Wisconsin BRFS Program is located in the Office of Health Informatics, Division of Public Health, Wisconsin Department of Health Services. BRFS is managed by the Wisconsin BRFS coordinator/project director in collaboration with the Centers for Disease Control and Prevention (CDC).

Sample Design

As of 2011, BRFS conducts both landline and cell phone-only interviews using separate sampling strategies. Currently, only BRFS landline data are available on WISH. Combined landline and cell phone data will be phased in as multiple years of jointly weighted data become available from CDC. In the interim, it should be kept in mind that certain prevalence estimates may be affected by the exclusion of cell phone interview data. See the CDC BRFSS Web site (exit DHS) for more information about combined landline/cell phone data and 2011 changes to the BRFSS weighting methodology.

The Wisconsin BRFS landline sample design is a stratified random sample based on Department of Health Services regions, county population size and, in some instances, the demographic composition of geographic areas. Weighted BRFS landline samples represent the adult population (ages 18+) living in households with a landline telephone. The landline sample excludes people living in institutions and other group quarters.

Within each sample stratum, CDC’s sampling contractor randomly generates batches of telephone numbers to create the sample. At the time of contact, one adult is randomly selected in each eligible household to complete the survey. The Centers for Disease Control and Prevention's (CDC's) BRFS protocol specifies that a new sample is opened and closed each month. Thus, the BRFS consists of 12 separate monthly samples that are combined to make the annual data set.

Recommendations for Analysis and Reporting of Results

Use of confidence intervals around BRFS survey estimates is strongly recommended. The Office of Health Informatics recommends basing all reported estimates on a minimum of 100 cases, or interviews; more than 100 cases is preferable. To increase the number of cases for an estimate using the BRFS Trend Data Module, use more than one year of data. For additional information about how to judge the reliability of estimates produced, examine the Relative Standard Errors that accompany the results.

Information Specific to the Trends Module

Analysis of sub-groups (e.g., women of childbearing age) requires consideration of issues concerning adequate sample size and appropriate population estimates.

For results that estimate population numbers and are based on more than one year of data, divide the "Estimated Number in the Population" shown in the results by the number of years to resolve the inflated numbers generated by multiple years of summed weights. This will produce an average estimated population number for the time period.

For example, selecting General Health Status for 2003-2005 shows an "Estimated Number in Population" of 7,198,600 who were in excellent/very good health in Wisconsin. Dividing this result by the number of years (3) yields 2,399,533.33, which rounds to 2,399,500. During 2003-2005 there were an estimated 2,399,500 adults in excellent/very good health in Wisconsin.

Information Specific to the All Counties Module

The All Counties module is designed to provide estimates for three-year time periods, and the time periods cannot be combined. The periods are fixed to assure an adequate number of cases per county for every county.

Weighting in the All Counties module is to individual county adult populations rather than the state adult population. The weighting difference is potentially important for small counties with population demographics that differ notably from the demographics of the state as a whole on factors related to health.

Last Revised: December 16, 2014