Published July 28, 2008
Fox News polls, conducted by Opinion Dynamics Corporation, are carried out via telephone using standard statistical methods to select a representative sample of registered voters.
Samples for Fox News polls are random digit samples of telephone numbers selected using the “probability proportionate to size” method, which means numbers from across the country are selected in proportion to the number of voters in each state.
A computer selects the first eight digits of an actual working number and then appends a two-digit random number to produce a random-digit dial (RDD) sample. An RDD sample allows for contacting both listed and unlisted landline numbers, as well as those households with new or changed numbers.
In order to ensure a distribution of ages and genders within households, the interviewer selects the respondent by asking to speak to the adult with the next birthday. Quotas are applied to ensure the sample mirrors the proportions of voters nationally. Specifically, the aim is for a gender split nationwide of 53 percent female and 47 percent male, as well as regional quotas.
In addition, since almost all Fox News polls are of registered voters, each respondent is screened to establish him/her as a registered voter who voted in the 2004 presidential election or has registered since then. In states without registration or same-day registration, the question asks about the frequency with which the individual votes. In the months prior to elections, interviewers sometimes screen for “likely voters” through a series of additional questions about past voting behavior and interest in voting in the current election.
The RDD selected phone numbers are sent to the interviewers through computer-assisted telephone interviewing (CATI) software. Both the software and human supervisors monitor each step of the interviewing process. While calls are automatically dialed, the system does not use predictive dialing so prospective respondents always find a live interviewer when they answer their phone.
A properly conducted national probability sample should accurately reflect national attitudes. However, particularly because FOX News polls are normally conducted over only two nights (limiting the opportunity for callbacks), some demographic deviation is possible. Opinion Dynamics Corporation compares each sample of respondents with a constantly updated database of demographic information about the national voting population and, when necessary, minor weights are applied to key demographic variables to bring the sample into conformity with the most reliable demographic profiles. FOX News polls are not weighted by political party.
For a sample of about 900 interviews, the error due to sampling is plus or minus three percentage points. For example, when the survey says “47 percent of voters . . .” then the probability is very high that no less than 44 percent and no more than 50 percent of all voters would have responded the same way.
In addition to sampling error, question wording and question order can influence poll results.