How does Gallup polling work?
Gallup polls aim to represent the opinions of a sample of people representing the same opinions that would be obtained if it were possible to interview everyone in a given country.
The majority of Gallup surveys in the U.S. are based on interviews conducted by landline and cellular telephones. Generally, Gallup refers to the target audience as "national adults," representing all adults, aged 18 and older, living in United States.
The findings from Gallup's U.S. surveys are based on the organization's standard national telephone samples, consisting of directory-assisted random-digit-dial (RDD) telephone samples using a proportionate, stratified sampling design. A computer randomly generates the phone numbers Gallup calls from all working phone exchanges (the first three numbers of your local phone number) and not-listed phone numbers; thus, Gallup is as likely to call unlisted phone numbers as listed phone numbers.
Within each contacted household reached via landline, an interview is sought with an adult 18 years of age or older living in the household who has had the most recent birthday. (This is a method pollsters commonly use to make a random selection within households without having to ask the respondent to provide a complete roster of adults living in the household.) Gallup does not use the same respondent selection procedure when making calls to cell phones because they are typically associated with one individual rather than shared among several members of a household.
When respondents to be interviewed are selected at random, every adult has an equal probability of falling into the sample. The typical sample size for a Gallup poll, either a traditional stand-alone poll or one night's interviewing from Gallup's Daily tracking, is 1,000 national adults with a margin of error of ±4 percentage points. Gallup's Daily tracking process now allows Gallup analysts to aggregate larger groups of interviews for more detailed subgroup analysis. But the accuracy of the estimates derived only marginally improves with larger sample sizes.
After Gallup collects and processes survey data, each respondent is assigned a weight so that the demographic characteristics of the total weighted sample of respondents match the latest estimates of the demographic characteristics of the adult population available from the U.S. Census Bureau. Gallup weights data to census estimates for gender, race, age, educational attainment, and region.
Read more about conducting polls in Gallup's longer paper, "How Are Polls Conducted?"