Types of medical studies

The core idea

In an experimental study, the researcher determines each participant's exposure by a controlled process, as in a clinical trial; in an observational study, they merely observe each participant's exposure and health condition.

And before reaching humans: basic research asks questions about theories and tests hypotheses and is the first step in any research, then translational research builds on its results to develop new ways of prevention, detection or treatment and test them.

Why it matters to you

The type of study determines the kind of question it can answer: a trial in which the researcher controls the exposure tests the effect directly, whereas an observational study reveals the association between exposure and disease.

This is why the GRADE approach starts rating the evidence from randomised trials with higher certainty than the evidence from non-randomised studies.

Study types in detail

Randomised controlled trial: it has at least one experimental treatment and one control treatment, and each participant's treatment is determined by lot; allocation by chance rather than by choice helps avoid bias.

The control group receives usual care or an inactive substance, and the experimental group's results are compared with it; in a double-blind study neither participants nor researchers know who is in which group.

Cohort study: compares disease rates between those who were exposed to a factor and those who were not; it may be prospective, following participants from its start, or retrospective, in which the exposure and outcomes had already occurred.

Case-control study: starts with a group with the disease and a comparison group without it, and compares their past exposures; the comparison group must be similar to the patients.

Cross-sectional study: measures exposure and health condition in a sample of people at the same time; suitable for describing prevalence, but weaker for analysis because it usually does not separate the factors for developing the disease from the factors for living with it.

Case reports: presentations of individual clinical cases that may be followed by evaluation studies.

Systematic review and meta-analysis: brings together all eligible studies and assesses them using explicit methods, and may combine their results statistically. See Evidence pyramid.

How to tell the study type

Read the "Methods" section of the study or its abstract: were participants allocated by lot? Was there a control group? Was the study blinded? And what steps were taken to reduce bias?

Or enter the study's DOI in the Study Reader to display the publication types the US National Library of Medicine has classified it under.

Common pitfalls

Confusing a "clinical study" with a "clinical trial": in an observational study, participants may receive interventions, but the researchers do not determine which intervention they receive.

Inferring cause from a cross-sectional study: because it measures the exposure and the condition together at one time, it is weaker for causal analysis.

Limits of this page

This page shows only the common types; other designs exist such as pragmatic trials, adaptive trials, equivalence trials and twin studies.

The type of study alone is not enough to judge its quality: sample size, length of follow-up and steps to reduce bias are all important.

Common questions