Cross-sectional studies are best suited when which condition holds?

• A. Exposure is time-variant and you aim to measure duration.
• B. Time-invariant exposures and you cannot determine whether the duration of the outcome is affected.
• C. You want to establish temporality.
• D. You want to study incidence.

Answer: (B)

Cross-sectional studies provide a snapshot of exposure and outcome at one point in time, so they’re most informative when the exposure status is stable over time. When the exposure is time-invariant, a single measurement reasonably reflects the person’s typical exposure, allowing you to examine associations with an outcome that exists at that moment. A key limitation is that you can’t determine whether the exposure preceded the outcome or whether the duration of the outcome is affected, because time ordering and duration aren’t observed over time in this design. If your goal is to know whether exposure changes over time, to assess duration of the outcome, or to measure new cases as they occur (incidence), longitudinal designs are needed. In light of that, time-invariant exposures with no interest in outcome duration fit cross-sectional studies best, which is why this option is the most appropriate.