Which study design characteristically uses surveys or questionnaires to collect data at a single point in time?

Correct answer: D. Cross-sectional study

• A. A case-control study starts with people who have the outcome and looks BACKWARD at exposure. It measures odds ratios, and while it may use questionnaires, its defining feature is the backward-looking comparison, not the single time point.
• B. An RCT allocates participants to an intervention or control and follows outcomes forward. It is an experimental design, not a survey.
• C. A cohort study follows an exposed and unexposed group FORWARD over time to measure incidence and relative risk.
• D. CORRECT. A CROSS-SECTIONAL study captures exposure and outcome SIMULTANEOUSLY in a defined population at one point in time, and the survey or questionnaire is its characteristic instrument. It is the standard design for measuring PREVALENCE — NHANES, the source of most US oral health prevalence data, is a cross-sectional survey. Its limitation is that it cannot establish temporality, so causal inference is weak.

Note: Anchor pairing: cross-sectional → prevalence; cohort → incidence and relative risk; case-control → odds ratio; RCT → causal efficacy.