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Epidemiological study designs
A study design is set by two questions – did the investigator assign the exposure (experiment) or only observe it (observational), and in which direction were data gathered: from exposure to outcome (cohort), from outcome back to exposure (case-control) or both at a single moment (cross-sectional).
Why it matters in the exam
Every exam has several items that describe a study in words or as a diagram and ask "what is the study type?", "what is the exposure and what is the outcome?", "which measure of association fits?" or "what is the main weakness of this design?". Recognize at once: a rare disease – case-control and an odds ratio (OR); a rare exposure or several outcomes – cohort and relative risk (RR); pooled country data – an ecological study with the ecological fallacy; a cross-sectional study – prevalence, with no certainty about time order. Further items deal with the controlled trial: randomization, double blinding, intention-to-treat analysis, the crossover design with its washout period, and the systematic review with meta-analysis and a funnel plot for publication bias. It is also worth remembering the order of evidence strength: a systematic review of randomized trials, a single randomized trial, cohort, case-control, cross-sectional and case reports, with the ranking also depending on the quality of execution. A historical cohort, which uses existing records of past exposure and follows people up to the present, is a cohort in every respect, and it is often the design chosen when an exposure is examined in a group of workers or soldiers.
Key points
- A cohort starts from exposure and yields an RR; a case-control study starts from disease and yields an OR.
- A cross-sectional study measures prevalence and cannot fix time order.
- An ecological study is exposed to the ecological fallacy.
- Randomization balances even unknown confounders, and intention-to-treat analysis preserves that balance.
- Meta-analysis pools studies, and a funnel plot checks for publication bias.
Summary table
| Design | Direction and measure | Main strength | Main weakness |
|---|---|---|---|
| Ecological | Group-level data, correlation | Cheap and quick, generates hypotheses | Ecological fallacy – a group-level link need not hold for individuals |
| Cross-sectional | Exposure and outcome at the same moment, prevalence and prevalence ratio | Describes burden and supports planning | No certainty which came first |
| Case-control | From outcome back to exposure, odds ratio (OR) | Efficient for rare disease and long latency | Recall bias and poorly chosen controls |
| Cohort (prospective or historical) | From exposure to outcome, incidence and relative risk (RR) | Clear time order, several outcomes from one exposure | Costly and long, loss to follow-up |
| Randomized controlled trial | The investigator assigns the intervention, RR and NNT | Balances even unknown confounders | Costly, selected population, not always ethical |
| Systematic review and meta-analysis | Pools the results of studies | A more precise estimate | Publication bias and heterogeneity between studies |
Free exam questions
?מהו סוג המחקר (the question as examined, in Hebrew)
- א. התערבות רנדומלי מבוקר
- ב. עוקבה
- ג. מקרה בקורת
- ד. מטה אנליזה
חוקרים ביקשו להשוות את שיעורי התחלואה בקורונה בנשים הרות שקיבלו חיסון לקורונה לבין נשים הרות שלא קיבלו חיסון לקורונה. החוקרים גייסו 7530 נשים הרות שחוסנו לקורונה ו 7530 נשים הרות שלא חוסנו לקורונה ועקבו אחריהן ע"י הזמנתן למרפאה במשך 70 ימים. כל הנשים ההרות ביצעו בדיקת PCR .לקורונה במהלך המעקב מה הוא סוג המחקר המתואר? (the question as examined, in Hebrew)
- א. מחקר עוקבה היסטורי
- ב. מחקר עוקבה פרוספקטיבי
- ג. ניסוי קליני רנדומלי מבוקר
- ד. מחקר מקרה ביקורת
In the full version for subscribers
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Sources
- Gordis, Epidemiology, 6th ed., 2019 — Observational studies; Randomized trials
- Rothman, Epidemiology: An Introduction, 2nd ed., 2012 — Types of epidemiologic studies
- Higgins et al., Cochrane Handbook for Systematic Reviews of Interventions, version 6, 2023 — Undertaking meta-analyses
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.