Most of our national surveys of the general public are conducted either online with our American Trends Panel or by telephone using a random digit sample of landline and cellphone numbers in the United States. Some of our surveys include additional, larger samples of subgroups, such as African Americans or young people (these are called “oversamples”). Pew Research Center also conducts international surveys that involve sampling and interviewing people in multiple countries. Lastly, we sometimes survey special populations, such as foreign policy experts, scientists or journalists. In all of our surveys, we use probability sampling to help ensure adequate representation of the groups we survey.
Learn more about random sampling by watching our Methods 101 video, “How can a survey of 1,000 people tell you what the whole U.S. thinks?”
Random digit dialing
The typical Pew Research Center telephone survey selects a random digit sample of both landline and cellphone numbers in all 50 U.S. states and the District of Columbia.
The design of the landline sample ensures representation of both listed and unlisted numbers (including those not yet listed) by using random digit dialing. This method uses random generation of the last two digits of telephone numbers selected on the basis of the area code, telephone exchange and bank number. A bank is defined as 100 contiguous telephone numbers, for example 800-555-1200 to 800-555-1299. The telephone exchanges are selected to be proportionally stratified by county and by telephone exchange within the county. That is, the number of telephone numbers randomly sampled from within a given county is proportional to that county’s share of telephone numbers in the U.S. Only banks of telephone numbers containing three or more listed residential numbers are selected.
The cellphone sample is drawn through systematic sampling from dedicated wireless banks of 100 contiguous numbers and shared service banks with no directory-listed landline numbers (to ensure that the cellphone sample does not include banks that are also included in the landline sample). The sample is designed to be representative both geographically and by large and small wireless carriers.
Both the landline and cell samples are released for interviewing in replicates, which are small random samples of the larger sample. Using replicates to control the release of telephone numbers ensures that the complete call procedures are followed for the entire sample. The use of replicates also ensures that the regional distribution of numbers called is appropriate. This also works to increase the representativeness of the sample.
When interviewers reach someone on a landline phone, they randomly ask half the sample if they could speak with “the youngest male, 18 years of age or older, who is now at home” and the other half of the sample to speak with “the youngest female, 18 years of age or older, who is now at home.” If there is no person of the requested gender at home, interviewers ask to speak with the youngest adult of the opposite gender. This method of selecting respondents within each household improves participation among young people, who are often more difficult to interview than older people because of their lifestyles.
Unlike a landline phone, a cellphone is assumed in Pew Research polls to be a personal device. Interviewers ask if the person who answers the cellphone is 18 years of age or older to determine if he or she is eligible to complete the survey. This means that, for those in the cell sample, no effort is made to give other household members a chance to be interviewed. Although some people share cellphones, it is still uncertain whether the benefits of sampling among the users of a shared cellphone outweigh the disadvantages.
Elites and other special populations
Representative surveys can be conducted with almost any population imaginable. It is common for surveyors to want to collect information from experts or elites in particular fields (such as policymakers, elected officials, scientists or news editors) and other special populations (such as special interest groups, people working in particular sectors, etc.). The principles of drawing a representative sample are the same whether the sample is of the general population or some other group. Decisions must be made about the size of the sample and the level of precision desired so that the survey can provide accurate estimates for the population of interest and any subgroups within the population that will be analyzed.
Some special challenges arise when sampling these populations. In particular, it may be difficult to find a sampling frame or list for the population of interest and this may influence how the population is defined. In addition, information may be available for only some methods of contacting potential respondents (e.g., email addresses but not phone numbers) and may vary for people within the sample. If most members in the population of interest have internet access and email addresses are available for contacting them, the web often provides a convenient and inexpensive way to survey experts or other special populations.
Pew Research Center occasionally conducts surveys of opinion leaders, especially those in public policy roles. The opinion of elites is often compared with that of the general public to better determine whether these groups have similar or different opinions. In addition, Pew Research Center has conducted several surveys designed to be representative of a special population, including surveying scientists, journalists, Muslim Americans, Howard Dean’s campaign supporters during the 2004 presidential primary campaign, political campaign consultants and constituent groups from a sample of federal agencies.
- Public and Scientists’ Views on Science and Society Jan. 29, 2015 (scientists)
- Public Sees U.S. Power Declining as Support for Global Engagement Slips Dec. 3, 2013 (foreign policy experts)
- A Portrait of Jewish Americans Oct. 1, 2013 (Jewish Americans)
- A Survey of LGBT Americans June 13, 2013 (LGBT adults)
- The Rise of Asian Americans June 19, 2012 (Asian Americans)
- War and Sacrifice in the Post-9/11 Era Oct. 5, 2011 (veterans)