Study Designs & Research Questions
Recognise how the way participants are selected and followed determines which questions a study can answer.
Quick revision ↓ Medical Dictionary
01Descriptive and analytical work
A case report describes one person's presentation; a case series describes several people without a comparison group. These reports can identify unusual events and generate hypotheses, but they cannot reliably estimate a treatment effect against an alternative. Analytical studies compare groups or relationships. Identify the research question before labelling a design: a sophisticated statistical analysis cannot create a missing comparator or establish a missing temporal sequence.
02Cross-sectional snapshots
A cross-sectional study measures characteristics within a defined population at a point or period in time. It is useful for estimating prevalence and exploring associations. When exposure and outcome are measured together, it may be unclear which came first. A survey linking poor sleep to depression cannot by itself establish whether sleep problems preceded depression, resulted from it, or share another cause.
03Cohort and case-control directions
A cohort groups people by exposure or other baseline characteristics and observes outcomes, prospectively or using existing records. A case-control study begins by selecting people with and without an outcome and compares prior exposures. The distinction depends on sampling, not simply whether data are old. Cohorts often estimate incidence directly; conventional case-control sampling does not preserve the population denominator needed for a direct risk estimate.
04Experiments and assignment
In a randomised trial, a chance mechanism assigns the intervention. This differs from observing treatments selected by clinicians or patients. Random allocation helps balance prognostic factors in expectation; it does not guarantee identical groups or prevent every later bias. Quasi-experimental studies evaluate interventions without individual random allocation and require careful attention to secular trends, concurrent changes and the credibility of the comparison.
05Fit design to purpose
Treatment effects, rare harms, prognosis, diagnostic accuracy and lived experience require different approaches. A well-conducted trial may answer an efficacy question but be too small or short for rare adverse effects. A qualitative interview study may be appropriate for understanding treatment barriers. A design hierarchy is a starting guide tied to the question, not a rule that any paper of a preferred design is trustworthy.
High-Yield Revision
- Identify who was selected first and why.
- Retrospective does not automatically mean case-control.
- Choose a design for the question and feasibility.
Common Pitfalls
- Association without a clear timeline does not establish cause.
- A design label alone does not establish low risk of bias.
References & Further Reading
Use the full methods and relevant chapters when appraising a paper. These sources support further study; the examples and questions above are original teaching exercises.
- Oxford CEBM — Critical appraisal tools
- Cochrane Handbook — Methods for evidence synthesis
- CDC — Analysing and interpreting epidemiological data
- ASA — Statement on p-values and statistical significance
- Oxford CEBM — Likelihood ratios
- Cochrane — GRADE and summary of findings
- WMA — Declaration of Helsinki, 2024 revision
- EQUATOR — Reporting guideline library
- NIH OBSSR — Mixed-methods research resources
- Institute for Healthcare Improvement — Model for Improvement