Three Main Types of NonExperimental Studies Crosssectional compare

Three Main Types of Non-Experimental Studies • Cross-sectional: compare prevalences or cross-sectional rates in sub-groups (no followup of individuals) • Cohort: recruit cohort sub-divide cohort individual follow-up over time compare incidences in sub-groups • Case-control: identify cases and a small number of non-cases compare exposure experience of cases and non-cases case-control-brief. ppt Intro to Case-Control Studies 1

For the sake of comparison, let’s first consider cohort designs… • Recruit cohort • Classify individual as exposed or non-exposed • Follow exposed and non-exposed sub-cohorts over time to determine incidence Closed population case-control-brief. ppt exposed sub-cohort → Incidenceexposed Compare Incidences non-exposed → Incidence nonsub-cohort exposed Intro to Case-Control Studies 2

…in contrast, case-control studies • Identifying cases • Identify a small sample of non-cases (“controls”) • Determine exposure histories of cases and controls All cases Population Very efficient for studying rare diseases Small number of noncases But can NOT estimate incidence case-control-brief. ppt Intro to Case-Control Studies 3

Case-Control Case-control studies cannot estimate incidence, BUT can estimate relative incidence (i. e. , relative risk) via the odds ratio (cross-product ratio): Exposed Non- exposed case-control-brief. ppt Cases Controls A 1 A 0 M 1 B 0 M 2 Intro to Case-Control Studies 4

Toxic Shock Syndrome and Tampon Use, Wisconsin, 1980 Tampon Use + Tampon Use − case-control-brief. ppt Cases 30 1 Controls 71 22 31 93 Intro to Case-Control Studies 5

Interpretation of the Odds Ratio • When the disease is rare, OR = RR (slightly more complicated than this, but nevermind) • The illustrative OR of 9. 3 suggests that tampon users had 9. 3 times the risk as non-tampon users • [The suspected brand of tampon has since been removed from the market] case-control-brief. ppt Intro to Case-Control Studies 6