In a survey study, a representative sample of individuals are asked to describe their opinions, attitudes, or behaviors. For surveys of behavior (eg, diet, exercise, smoking), authors should provide evidence that the survey instrument correlates with the actual, observed behaviors of a similar sample of individuals. That is, the survey instrument should have been shown to have validity. If the survey instrument is different in any way from that given to the previous validation sample (eg, wording, order, or omission of questions), then it may no longer be a valid measure of those behaviors.
For surveys, as for other studies, it is critical to describe explicit inclusion and exclusion criteria, as well as how and when individuals left the study once they were initially identified. Flow diagrams can be a useful way of presenting this information. There is currently no standard reporting format for survey studies, however, and authors have usually reported no more than a single response rate for their survey. To address this situation, the American Association for Public Opinion Research (AAPOR) has published a set of expanded definitions.36 The AAPOR document defines response rate as “the number of complete interviews with reporting units divided by the number of eligible reporting units in the sample.” The document points out that this general definition allows for at least 6 different ways of actually computing this statistic, depending on how the numbers of “complete interviews” and the “number of eligible reporting units” are defined. The document goes on to define 4 possible equations for cooperation rates (the proportion of all cases interviewed of all eligible units ever contacted), 3 equations for refusal rates (the proportion of all cases in which a housing unit or respondent refuses to do an interview), and 3 equations for contact rates (the proportion of all cases in which some responsible member of the housing unit was reached by the survey). Thus, authors should be clear about how they assigned individuals to categories and which categories they used to compute these statistics.
The AAPOR document defines specific reporting procedures for the 3 most common survey designs: random-digit-dial telephone surveys, in-person surveys, and mail surveys. Future updates of the AAPOR document will discuss Internet-based surveys. As with observational studies, meta-analyses, and cost-benefit studies, there are currently no universally agreed-upon reporting criteria for survey studies.
Survey studies may be either longitudinal (the same respondents are surveyed at several time points) or cross-sectional. Causality may be cautiously inferred from longitudinal surveys, but never from cross-sectional surveys. Case-control studies (see 20.3.2) and cohort studies (see 20.3.1) may exclusively use survey methodology to obtain their dependent variables, and thus in practice the distinction between observational studies and survey studies may be nuanced.