Methodology

This study is based on a telephone survey of the Illinois adult population, aged 18 and older. Interviewers at the Public Opinion Laboratory at Northern Illinois University completed 1309 Interviews in November of 2004 through December of 2004. Interviews were conducted and responses recorded using a computer assisted telephone interview system.

The Illinois Policy Survey uses a disproportionately stratified sample, divided into six geographical areas: Chicago, the balance of Cook County, five collar counties (DuPage, Kane, Lake, McHenry and Will), the rest of Northern Illinois, Central Illinois and Southern Illinois. Sample sizes in the geographical areas ranged from 201 to 242.

A two-stage sampling strategy was used. For the first stage, telephone numbers were drawn with a known probability of selection within regions. Both listed and unlisted numbers were included in the sample at this stage. The second stage of sampling was carried out when an interviewer reached an eligible household. An eligible household was a non-institutional residence in Illinois which was the home of at least one adult
18 years of age or older. After reaching an adult informant in a household, interviewers gave a brief explanation of the survey and the purpose of the selection process. The household member to be interviewed was randomly selected from among the adults who were at home.

Data were weighted to adjust for disproportionality in the sampling. Unequal probabilities of selection at each stage of sampling were accounted for in the initial weight. A post-stratification adjustment to the initial weight was calculated by using a six by two by six groupings of age groups by sex, by region using current population
estimates from EASI data. The weighted sample was adjusted back to the original sample size for analysis.
The sampling and weighting procedures were designed to provide a representative sample of the adult population of Illinois that is accessible by telephone. This year's survey was conducted in both English and Spanish. Thirty-seven of the final interviews were conducted in Spanish. The conservative 95% confidence interval for a sample of 1309 individuals is plus or minus 3%. This means there is a 95% likelihood the
population percentages will fall within 3 percentage points of estimates based on the sample percentage. Confidence intervals for subgroups of the sample will be larger, and will vary with the size of the subgroup.