The sample of land-line telephone exchanges called was randomly selected by a computer from a complete list of more than 72,000 active residential exchanges across the country. The exchanges were chosen so as to ensure that each region of the country was represented in proportion to its share of all telephone numbers. Within each exchange, random digits were added to form a complete telephone number, thus permitting access to listed and unlisted numbers alike. Within each household, one adult was designated by a random procedure to be the respondent for the survey. To increase coverage, this land-line sample was supplemented by respondents reached through random dialing of cellphone numbers. The two samples were then combined and adjusted to ensure the proper ratio of land-line-only, cellphone-only and dual phone users. Interviewers made multiple attempts to reach every phone number in the survey, calling back unanswered numbers on different days at different times of both day and evening. The combined results have been weighted to adjust for variation in the sample relating to geographic region, sex, race, Hispanic origin, marital status, age, education and number of adults in the household. Respondents in the land-line sample were also weighted to take account of the number of telephone lines into the residence. Self-identified Republicans, Democrats and independents were also adjusted to their average proportion in the most recent seven polls by The Times and CBS News. In theory, in 19 cases out of 20, overall results based on such samples will differ by no more than 3 percentage points in either direction from what would have been obtained by seeking to interview all American adults. For smaller subgroups, the margin of sampling error is larger. Shifts in results between polls over time also have a larger sampling error. In addition to sampling error, the practical difficulties of conducting any survey of public opinion may introduce other sources of error into the poll. Variation in the wording and order of questions, for example, may lead to somewhat different results. Michael R. Kagay of Princeton, N.J., assisted The Times in its polling analysis. Complete questions and results are available at nytimes.com/polls.
