What it shows
Three observational designs on one timeline, with smoking as the exposure and lung cancer as the outcome: a prospective cohort study, a retrospective cohort study and a case-control study.
In each row an orange arrow shows the direction of inquiry, from the groups the study starts with to what it then measures. Gray boxes are exposure groups and white boxes are outcome groups.
How to read it
The dashed line at 0 is when the study starts. The axis runs from 20 years before it to 20 years after.
A prospective cohort starts at 0 with smokers and nonsmokers and follows them forward to see who develops lung cancer.
A retrospective cohort also runs from exposure to outcome, but both are already in records: it starts with smokers and nonsmokers recorded 20 years earlier.
A case-control study runs the other way. It starts with people who have lung cancer (cases) and people who do not (controls), then looks back at each group’s smoking history.
What each design compares
- Prospective cohort: lung cancer in smokers against nonsmokers, as a risk ratio or an odds ratio.
- Retrospective cohort: the same comparison from past records, again as a risk ratio or an odds ratio.
- Case-control: smoking history in cases against controls, as an odds ratio.
- The 20-year span is illustrative and does not come from a particular study.
Ways to use it in teaching
Ask students to place a short published abstract on the right row by asking one question: did the study start from the exposure or from the outcome?
Use it to explain why a case-control study cannot calculate a risk ratio directly (CDC), so it reports an odds ratio, while a cohort study can.
For a rare outcome like lung cancer, ask how many people a cohort would need to follow, and why a case-control design is the cheaper first step.
For how a cohort design compares with other ways of studying change over time, see our guide to longitudinal studies.
Sources
- Case–control study. (n.d.). In Wikipedia. Retrieved October 19, 2026, from https://en.wikipedia.org/wiki/Case%E2%80%93control_study
- Centers for Disease Control and Prevention. (2006). Principles of epidemiology in public health practice: An introduction to applied epidemiology and biostatistics (3rd ed.). https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/index.html
- Cohort study. (n.d.). In Wikipedia. Retrieved October 19, 2026, from https://en.wikipedia.org/wiki/Cohort_study
- Retrospective cohort study. (n.d.). In Wikipedia. Retrieved October 19, 2026, from https://en.wikipedia.org/wiki/Retrospective_cohort_study
