The document ftp.cdc.gov/pub/Data/Brfss/userguide.pdf has detailed information required of Illinois and other states participating in the BRFSS; including survey protocol (page7), questionnaire development (pages 24-44), and survey methodology (pages 45-55).

Using a scientifically designed method, a random sample of telephone numbers is selected each month for telephone interviews. The Illinois BRFSS program uses a dual questionnaire procedure to collect data on a variety of health-related subjects. This technique allows Illinois to provide useful information to more health programs. This sample is then split in two with each half being asked different versions of the survey. Both Illinois versions use the same core questions as the rest of the states, but different state-added questions are asked of each group. The ICBRFS combines elements from the dual questionnaires into one survey instrument that includes the same core topics. Procedures used to conduct interviews are the same for all questionnaire forms. Interviewers follow carefully controlled procedures to assure that the collected data meet standards of acceptability. Once all the data for a year are collected, the dataset is edited in preparation for weighting.

Data-users may select frequency or cross-tabulation data for any of three categories: Illinois, strata, or county. Within the strata, any of five strata may be selected: Chicago; suburban Cook County; collar counties; urban counties; or rural counties. The collar counties include DuPage, Kane, Lake, McHenry and Will. Urban counties include Champaign, DeKalb, Kankakee, Kendall, McLean, Macon, Madison, Peoria, Rock Island, Sangamon, St. Clair, Tazewell and Winnebago. The remaining 83 counties are grouped as rural counties.

Weighting is a process that improves precision of prevalence estimates by performing three functions; it equalizes probability of being selected for the survey, it corrects for variation of age/race/sex groups between the sample and the population, and it permits generalization of the survey data to the entire population.

An important point to note is that the Illinois BRFSS data were weighted with a process that took the elements of the split sample and dual questionnaire into consideration. By excluding the split sample factor, other sources may not present Illinois BRFSS data with the proper weighting factors.

For more information about the BRFSS program at CDC, visit that agency's website at www.cdc.gov/brfss.