Clinical Study

A clinical study is the umbrella term ClinicalTrials.gov and the U.S. National Library of Medicine (NLM, part of NIH) use for any research study involving human volunteers (participants) that is intended to add to medical knowledge. Under this definition, every clinical study is one of two subtypes: an interventional study — commonly called a clinical trial — in which participants are prospectively assigned, according to a protocol, to receive a specific intervention (a drug, device, procedure, or behavioral change) so its effect can be evaluated; or an observational study, in which participants are assessed for health-related outcomes or characteristics as they occur naturally, without the investigator assigning any intervention. "Clinical study" is therefore broader than "clinical trial": every clinical trial is a clinical study, but not every clinical study is a clinical trial. This is a separate axis of classification from NIH's own narrower, funding-and-policy-consequential test for whether a specific study is an "NIH-defined clinical trial" (prospective assignment to an intervention to evaluate its effect on a health-related biomedical or behavioral outcome) — a study can be a clinical trial in the general ClinicalTrials.gov sense while the NIH four-question test is what actually determines registration, reporting, and funding-opportunity obligations for NIH-funded work.

ByClinicalTrials.gov / U.S. National Library of Medicine (NIH)

· Last updated 17 Jul 2026

Worked examples

Is an instance

A study that randomly assigns participants to receive either a new antihypertensive drug or a placebo and measures the change in blood pressure is a clinical study of the interventional subtype — in common usage, a clinical trial.

Is an instance

A study that enrolls an existing cohort of patients already receiving standard-of-care therapy and follows them over time to observe how their outcomes evolve, without the investigator assigning any intervention, is a clinical study of the observational subtype.

Looks similar, but isn't

Not an instance

A laboratory bench study using only cell lines, tissue samples, or animal models, with no human participants, is not a clinical study under this definition — ClinicalTrials.gov's definition specifically requires human volunteers.

Not an instance

A purely retrospective chart review or registry analysis of already-collected clinical records, where no participant is prospectively enrolled or assessed going forward, is commonly described as clinical or health-services research but does not fit the ClinicalTrials.gov "clinical study" registration scope, which is built around prospectively conducted interventional and observational studies.

“Clinical study” and “clinical trial” are routinely used interchangeably in casual conversation, but on ClinicalTrials.gov and in U.S. National Library of Medicine (NLM) guidance they are not synonyms — one is the umbrella category, the other is a subtype of it. Getting this distinction right matters for anyone searching a registry, writing a protocol, or trying to work out whether a given study needs to register and report results.

The two subtypes of a clinical study

Per ClinicalTrials.gov and NLM’s own guidance, a clinical study is any research study involving human volunteers that is intended to add to medical knowledge. Every clinical study registered on ClinicalTrials.gov falls into one of two subtypes:

Interventional study (clinical trial) — participants are prospectively assigned, according to a protocol, to receive a specific intervention (a drug, biologic, device, procedure, or behavioral/dietary change), and the study evaluates that intervention’s effect.
Observational study — participants are identified as belonging to study groups and are assessed for biomedical or health outcomes, but the investigator does not assign any intervention. Cohort studies, case-control studies, and cross-sectional studies are common observational designs.

In other words: every clinical trial is a clinical study, but not every clinical study is a clinical trial. An observational cohort study is a clinical study and is registrable on ClinicalTrials.gov, but it is not a clinical trial in the platform’s own terminology, because no intervention is assigned.

Why the terms get conflated

The conflation is understandable: in everyday and even much scientific writing, “clinical trial” is often used loosely to mean any prospective clinical research, and news coverage of medical research defaults to “trial” regardless of design. It’s also complicated by the fact that several major bodies define “clinical trial” slightly differently for their own regulatory purposes — ICMJE’s definition is deliberately broad and phase-agnostic, while NIH applies a narrower four-question operational test (see below), and FDAAA 801 defines a further, narrower subcategory (the “applicable clinical trial”) that triggers a legal registration and results-reporting mandate. None of those definitions change the ClinicalTrials.gov-level distinction described above; they operate one level down, further classifying which studies within the “interventional/clinical trial” bucket trigger which specific obligations.

How this relates to NIH’s clinical trial definition

NIH funding policy adds a second, separate classification question on top of the study-type distinction above: whether a specific study meets NIH’s own operational definition of an NIH-defined clinical trial. That test requires a “yes” to all four of NIH’s screening questions — human participants, prospective assignment to an intervention, evaluation of the intervention’s effect, and a health-related biomedical or behavioral outcome — and it determines which NIH funding opportunities are open to the study, whether it must register and report results on ClinicalTrials.gov under NIH policy, and whether staff need Good Clinical Practice (GCP) training. See What Is a Clinical Trial? The NIH Definition Explained for the full four-question test and worked examples of studies that do and don’t qualify.

The relationship between the two classifications: a study must first be an interventional study (a clinical trial in the ClinicalTrials.gov sense) before NIH’s four-question test is even relevant — observational studies never meet NIH’s definition, because they fail the “prospectively assigned to an intervention” question by design. Among interventional studies, NIH’s test then determines which specific ones are subject to NIH’s own clinical-trial-specific funding and reporting machinery.

Why the distinction matters in practice

Registry searching: filtering ClinicalTrials.gov by “Study Type” (Interventional vs. Observational) is the fastest way to separate trials from observational research when searching for prior or ongoing work on a topic.
Protocol design and reporting standards: interventional studies typically follow CONSORT-family reporting guidelines, while observational studies follow STROBE — the applicable reporting standard depends on which subtype of clinical study is being conducted.
Registration and results-reporting obligations: registration requirements (ICMJE, FDAAA 801, NIH policy) are calibrated primarily around interventional studies/clinical trials; observational studies can be registered on ClinicalTrials.gov but are not generally subject to the same legal results-reporting mandates. See Clinical Trial Registration for how that obligation is scoped.
Ethical and regulatory oversight: both subtypes require IRB/ethics committee review as human subjects research, but interventional studies typically carry additional oversight (data safety monitoring, informed consent for randomization) tied to the assignment of an intervention.

Frequently asked questions

Is an observational study a clinical trial?

No. An observational study is a clinical study, but it is not a clinical trial — the investigator does not assign an intervention, which is the defining feature that makes an interventional study a clinical trial in ClinicalTrials.gov’s terminology.

Does ClinicalTrials.gov only list clinical trials?

No. ClinicalTrials.gov registers both interventional studies (clinical trials) and observational studies, and labels each record’s “Study Type” accordingly — the registry’s scope is “clinical study,” not just “clinical trial.”

Which term should I use in a protocol or manuscript?

Use “clinical trial” only when describing research in which participants are prospectively assigned to an intervention; use the broader “clinical study” when referring generically to human-subjects research that may include both interventional and observational designs, or when you haven’t yet specified the design type.

For the general structural taxonomy of research designs (cohort, case-control, cross-sectional, RCT, and more) independent of the clinical-research context specifically, see Research Study Types.

Use in your systems

JATS XML <role> element

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Schema.org DefinedTerm (JSON-LD)

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