cross-sectional study

Meaning: A cross-sectional study collects data from a sample at a single point in time, usually comparing groups that differ in a characteristic such as age, income, or exposure. Researchers do not manipulate variables or follow people afterward. Because the exposure and the outcome are measured together, the design generally cannot show which came first, so it can reveal associations but not causation.

Equation: Prevalence = number of existing cases at that time ÷ number of people in the sample (unitless proportion, often expressed per 100 or per 1000).

On the MCAT: A passage describes surveying 20-, 40-, and 60-year-olds in the same year and finding different scores on a measure. You may be asked to name the design, give a strength (fast, inexpensive, no attrition) or a limitation (no temporal order, confounding, cohort effects). Expect a comparison with a longitudinal study, which follows the same individuals over time. That design tracks change within persons but costs more time and money and loses participants through attrition.

Common confusion: Differences between age groups in a cross-sectional study are not automatically developmental change. The groups were born in different eras, so a cohort effect (shared generational experience) can explain the pattern. Following the same people over time, as a longitudinal design does, measures within-person change directly.