What is a key limitation of cross-sectional studies?

• A. They assess exposure and outcome at one time, so temporality and causality cannot be established; susceptible to prevalence incidence bias.
• B. They track changes in health status over time to establish causality.
• C. They always provide strong evidence for causality.
• D. They avoid any bias due to sampling.

Answer: (A)

Cross-sectional studies provide a single-time snapshot of exposure and health status. Because both pieces of information are collected at the same moment, you can’t tell which came first—whether the exposure preceded the outcome or the outcome influenced exposure. That lack of temporality makes it impossible to infer causality from the observed association. These studies can describe how common a condition is and generate hypotheses, but they don’t establish that one factor causes another. They’re also prone to prevalence–incidence bias: depending on how long the condition lasts and how survival affects who is present at the time of the survey, the observed associations can be distorted. In addition, because they don’t follow people over time, they can’t measure incidence or track changes, and various sampling biases can affect results.