Methodology
To create a demographic and health profile of our adult membership in Northern California, we mail surveys to stratified random samples of about 42,000 adult members. Stratification is based on medical center, gender, and age. From each of our 19 medical center service populations, we randomly select approximately 2,200 members from four age groups (20-44, 45-64, 65-74, and 75 and older). To ensure that adequate numbers of racial and ethnic groups are represented, we sample approximately 2,500 members from our Oakland, Hayward, Richmond, and San Francisco medical center service populations--our most ethnically diverse service populations. We also oversample members aged 65 and over, striving to obtain data from 5,000 seniors in order to learn as much as possible about the characteristics of this group that is a major consumer of health care. Because the survey is currently only available in English due to cost considerations, the sample is restricted to members not known to require an interpreter or written materials in a language other than English.
We mail out the survey in mid-April, followed by up to two more mailings three to four weeks apart to enhance the final response rate.
After the data are collected, we assign survey respondents a weighting factor based on the number of members of their sex and age group (5-year intervals) in their medical center service population. For example, data for 35 year old and 50 year old male respondents drawn from the Oakland medical center service population might be weighted to represent 241 and 183 men, respectively, whereas data for male respondents of these ages drawn from the Kaiser Permanente-Sacramento service population might be weighted to represent 344 and 386 men, respectively. We use the weighted data, which reflect the actual age, gender, and geographic composition of the membership, to estimate the demographic characteristics and prevalence of different health conditions and health risks in the region and service populations.