Cross-sectional studies Representative sample Select random sample of population Study population Identify exposure status and disease status within sample. Disease No disease Exposed Not exposed One point in time – non-directional (possibly backward). Easy and economical. Relatively quick. Can’t say much at all about causality. Great as first, quick look.

Cross-sectional studies Cross-sectional studies investigate disease risk at a single point in time. future     present past With disease (case)No disease

disease-free controls Case-control studies Cases Deliberately recruit more cases than in a random sample Study population Controls Select similar disease-free controls Exposed Not exposed Compare exposure status among cases & controls

   Recruiting many disease cases increases the Case-control studies Recruiting many disease cases increases the study’s ability to identify risk factors. future    present Some case-control studies examine past exposures and are also known as retrospective studies. Exposure past

Cohort (longitudinal) studies Recruit random sample of people without the disease People without the disease Study population Record exposure & disease over time Time 1 Disease No disease Exposed Not exposed Population Time 2 Etc.

Cohort (longitudinal) studies     future Follow-up         Recruit- ment present Cohort studies select individuals and observe how their exposure and disease status changes over time. They are sometimes known as prospective studies. past