Surveys are instruments that researchers use to measure attitudes, tastes, viewpoints, and/or facts from a specific population. Most populations (or groups) are too large and widespread geographically to allow researchers to obtain information on each group member. To compensate, researchers have devised several methods by which they make inferences about a population by using information gathered from a selected sample of the population.
Various government agencies (including the U.S. Census Bureau) and academic disciplines (including economics, political science, and sociology) make frequent use of survey sampling to better understand the prevailing characteristics of specific populations. This entry examines survey sampling through the two most common formats in which it is conducted: questionnaires and interviews. Before moving to this discussion, however, it is worthwhile to describe the central methodology that makes survey sampling of populations so effective—drawing the actual sample.
The most commonly used procedure in drawing a survey sample is the simple random sampling (SRS) method. In order to make effective inferences about a population, survey researchers must be confident that the sample they are using is representative of the population in which they are interested. If the sample is not representative then bias (the misrepresentation of a population’s characteristics) can result. SRS provides assurance that the sample represents the population because each sampling unit (a person) has an equal probability of being selected to participate in the survey.
Though an equal probability of random selection helps mitigate response bias in survey samples, most populations of interest to researchers have such significant variation that SRS alone does not provide enough confidence that the sample is truly representative of a population. In order to address population variation, national polling techniques, such as those practiced by Gallup, require an initial stratification of the population before the random sample is drawn. Stratification is the division of a population into homogenous groups according to a specific set of dimensions or stratums (e.g., geographic location, age, sex). Once the population is divided in this manner, SRS is applied to each stratum. In most cases, survey researchers draw a proportionate stratified sample, which helps to keep the sampling units from each stratum closely resembling their proportion in the overall population. Most surveys, whether delivered in questionnaire or interview format, make use of a stratified SRS methodology.
Questionnaires are impersonal surveys used to collect data from respondents that have been targeted as part of a sample. Questionnaire surveys have traditionally been delivered through the mail to sample respondents. The growing popularity of e-mail in the late 1990s allowed researchers to employ this method of delivery more frequently. Despite its growing popularity, however, most survey questionnaires do not use the e-mail method because of the relatively large number of people with limited or no e-mail access as of the early twenty-first century.
The primary advantage of questionnaires, regardless of delivery method, is that they are relatively low cost. In stark contrast to interviews, questionnaires do not require the assistance of trained staff. In addition, because it costs the same to mail a survey three miles or three thousand miles, there are financial advantages to conducting national surveys. Access to bulk mail rates can provide even greater savings for researchers. Another advantage to questionnaires is that they reduce bias errors that personal interviewers may introduce. Whenever one person is talking to another to obtain information, an interpersonal dynamic is introduced that can alter the way a respondent answers questions. Since questionnaires are delivered through paper or computer, this missing human element is a welcome absence. Finally, questionnaires provide greater anonymity, in large part because there is no interviewer aware of respondent identity.
There are also disadvantages to questionnaires. Primary among these is that survey questions must be fairly simple so as to be understood by the vast majority of intended respondents. If questions are too complex or vague, respondents may miss the point of a question entirely, thereby introducing response bias. Also problematic is the inability of researchers to probe respondents for more specific information on topics. Question answers are final. In addition, researchers have no control over who actually completes the questionnaire since they have no direct contact with the respondent. Finally, researchers face low response rates (20 percent to 40 percent) when using questionnaire-based surveys. Most published research using data collected from mail surveys reports a response rate between 20 to 30 percent, although the rate is sometimes higher for targeted populations. The Internet’s popularity has helped to increase response rates by allowing researchers to follow-up with respondents through a hybrid approach in which both mail and e-mail requests for questionnaire completion are transmitted to respondents.
Personal interviews form the backbone of modern opinion polling. Usually conducted by a team of well-trained interviewers, these interviews enable polling companies to receive respondent data in a much shorter time frame than is required for questionnaires. Most interviews in opinion polling are of the schedule-structured variety, in which all respondents are asked the same questions, in the same order, in the same way so as to reduce response bias. Other interview forms include the focused (which allows the interviewer to ask probing questions depending upon how a respondent answers) and nondirective (in which the interviewer provides little structure or form). The focused and nondirective approaches are usually employed by academics focusing on a small sample of respondents in order to build empirical theories.
The primary advantage of the structured interview is that it gives researchers better control over the interview situation. The most direct improvement of interviews over questionnaires is that it is unlikely someone other than the respondent will provide question responses. Concomitantly, interviews have a much higher response rate than questionnaires (usually 95%), adding to their usefulness when time is of the essence. Of course, there are disadvantages, not the least of which is the interview bias referenced above. Cost is also a disincentive.