This Pew Internet & American Life Project report is based on the findings of a daily tracking survey on Americans’ use of the Internet and an online survey about Internet health resources.
Telephone interviews were conducted by Princeton Survey Research Associates between November 25 and December 22, 2002, among a sample of 2,038 adults, 18 and older. For results based on the total sample, one can say with 95% confidence that the error attributable to sampling and other random effects is plus or minus 2 percentage points. For results based Internet users (n=1,220) the margin of sampling error is plus or minus 3 percentage points. In addition to sampling error, question wording and practical difficulties in conducting telephone surveys may introduce some error or bias into the findings of opinion polls.
The sample for this survey is a random digit sample of telephone numbers selected from telephone exchanges in the continental United States. The random digit aspect of the sample is used to avoid “listing” bias and provides representation of both listed and unlisted numbers (including not-yet-listed numbers). The design of the sample achieves this representation by random generation of the last two digits of telephone numbers selected on the basis of their area code, telephone exchange, and bank number.
New sample was released daily and was kept in the field for at least five days. This ensures that complete call procedures were followed for the entire sample. Additionally, the sample was released in replicates to make sure that the telephone numbers called are distributed appropriately across regions of the country. At least 10 attempts were made to complete an interview at every household in the sample. The calls were staggered over times of day and days of the week to maximize the chances of making contact with a potential respondent. Interview refusals were re-contacted at least once in order to try again to complete an interview. All interviews completed on any given day were considered to be the final sample for that day. The overall response rate was 32.8%.
Non-response in telephone interviews produces some known biases in survey-derived estimates because participation tends to vary for different subgroups of the population, and these subgroups are likely to vary also on questions of substantive interest. In order to compensate for these known biases, the sample data are weighted in analysis. The demographic weighting parameters are derived from a special analysis of the most recently available Census Bureau’s Current Population Survey (March 2001). This analysis produces population parameters for the demographic characteristics of adults age 18 or older, living in households that contain a telephone. These parameters are then compared with the sample characteristics to construct sample weights. The weights are derived using an iterative technique that simultaneously balances the distribution of all weighting parameters.
An online survey consisting of 20 questions was hosted by Princeton Survey Research Associates. Respondents were primarily recruited from announcements posted on Braintalk.org (hosted by the Department of Neurology, Massachusetts General Hospital); ACOR.org (the Association of Online Cancer Resources); DrGreene.com (a pediatric Web site); and on the Pew Internet Project’s own site. An announcement was also printed in a syndicated newspaper column entitled “The People’s Pharmacy.” In addition to the “official” announcements, individual Internet users posted links to the survey in a multitude of personal emails, listserv discussion groups, and other health-related Web sites.
Respondents were invited to complete the multiple-choice questions and most used the open-ended text boxes to provide more detail. In all, 1,971 individuals’ responses were collected and transmitted to reviewers as three spreadsheets. Follow-up interviews with 19 respondents were completed via email.