Lesson 3 · Observational Designs · Step 2 of 6
Cross-sectional studies and the problem of timing
A cross-sectional study measures exposure and outcome at the same moment: walk into a village and record everyone's blood pressure and malaria status in one day. It is quick and cheap, and a single survey round can show stark inequalities, like the 17.6% of mothers in Ethiopia's pastoral regions practicing exclusive breastfeeding. The weakness is timing. If depressed people exercise less, the snapshot can't say which came first. Cross-sectional studies measure prevalence, the share who have a condition now. Incidence, the rate of new cases, needs follow-up over time.
Also covered: prevalence vs incidence · temporality · outbreak investigation · person-time
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Lesson 3 · 6 steps
1What descriptive designs can and cannot tell us7:162Cross-sectional studies and the problem of timing4:513Case-control studies: starting from the outcome6:214Cohort studies: following people forward in time6:395Controlling confounding in observational data7:496Choosing and reading an observational design6:45
Research Designs · lesson 3 of 5