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ABCNEWS polls of the general population are conducted by telephone among a random national sample of adults age 18 and over. Sampling and data collection for most of these polls is conducted by TNS Intersearch of Horsham, Pa.
Sampling A sample of households in the continental United States is selected via random digit dialing (RDD) procedures, to insure that all possible listed and unlisted phone numbers are included with equal probability of selection. RDD samples are produced using the Genesys Sampling System from a sampling frame that includes all active telephone area codes and exchanges. Before sampling, exchanges are stratified into nine regions as defined by the U.S. Census Bureau: New England, Middle Atlantic, East North Central, West North Central, South Atlantic, East South Central, West South Central, Mountain, Pacific. Counties within each region are then classified as metropolitan or non-metropolitan, using definitions established by the U.S. Office of Management and Budget. Within each of the resulting 18 strata, exchanges are grouped by state and then sorted by median county income as reported by the Census. These steps help ensure representativeness. Sampling then occurs in three stages. First, a systematic random sample of telephone exchanges is selected within each stratum, by taking every nth exchange. Next, telephone banks (the first two digits of the four-digit suffix) with more than one residential listing assigned in white pages directories are classified as working banks. A sample of four-digit suffixes is randomly selected from among working banks in each exchange, resulting in a self-weighting sample of households. This sample is checked against a database of known business telephone numbers to reduce business listings in the sample. The sampled phone numbers are pre-dialed via a non-ringing auto-dialer to reduce dialing of non-working numbers. The third stage of sampling is respondent selection within the household, accomplished by last-birthday selection. Interviewers ask to speak to the household member age 18 or over at home who's had the last birthday. To compensate for the fact that women tend to be easier to reach, in-house selection is stratified by sex, with interviewers asking to speak with the male household member 75 percent of the time and the female 25 percent of the time. If a person of the selected sex is unavailable, the interviewer asks to speak with the person of the other sex who had the last birthday. Interviewing Phone numbers are released for interviewing in replicates by region to allow for sample control. In multi-night polls, numbers are called multiple times during the field period. Interviews are conducted via computer-assisted telephone interviewing (CATI). The professional interviewers employed by TNSI, and their supervisors, are extensively trained in interviewing practices, including techniques designed to achieve the highest possible respondent cooperation. Weighting Final data are weighted using demographic information from the Census to adjust for sampling and nonsampling deviations from population values. Respondents customarily are classified into one of 48 cells based on age, race, sex and education. Weights are assigned so the proportion in each of these 48 cells matches the actual population proportion according to the Census Bureau's most recent Current Population Survey. Sampling Error
Poll results may deviate from full population values because a sample, not a census, is interviewed. Sampling error can be calculated when probability sampling methods, such as those described here, are employed. The standard formula to calculate sampling error at the 95 percent confidence level is: (SQRT(.25/sample size))*1.96. There can be other sources of differences in polls, such as the wording and order of questions. |
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