Summary characteristics of cohort, case-control, and cross sectional studies.25
| Characteristics | Cohort study | Case control study | Cross sectional study |
|---|---|---|---|
| Population at risk | Defined at the beginning | May be undefined | Begins with a defined population |
| Cases | Not selected but ascertained by surveillance | Selected by investigator from an available pool of patients | Not selected but ascertained by a single examination of the population |
| Controls | Not selected, evolve naturally | Selected by investigator to resemble cases | Include those free from disease at the single examination of the population |
| Exposure | Measured before development of disease | Measured, reconstructed, or recollected after development of disease | Measured at same time as disease |
| R or I and RR | Measured directly | R and “I” cannot be measured directly; RR of exposure estimated as OR | R and I cannot be measured directly; RR of exposure estimated as OR |
R - risk, RR - relative risk, I - incidence, OR - odds ratio25