Provenance

Multiple reports of one study: how reviews link them

Last checked 7 November 2026 · 6 sources

Short answer

One study can appear in several journal articles, conference abstracts and registry entries. Counted separately, they make the evidence look larger than it is. The Cochrane Handbook makes the study, not the report, the unit of a review, and its mandatory standard C42 says it is wrong to treat multiple reports of one study as multiple studies ¹. Trial registration numbers are the first linking criterion it lists ¹. ICMJE asks for a visible reference to any earlier publication ³.

Unit of interest: The study, not the report ¹
Cochrane standard: C42, Collating multiple reports, mandatory ¹
First linking criterion: Trial registration numbers ¹
Reporting guideline: PRISMA 2020, with template flow diagrams ²
Risk named by Cochrane: Undetected duplicate publication can double count participants ¹

The problem

The Cochrane Handbook says a study may be reported in several sources, and a report may describe more than one study ¹. It says it should never be assumed that there is a single report for each study ¹.

Linking is not always easy. Cochrane says multiple sources about the same trial may not reference each other, may not share common authors, or may report discrepant information about design, outcomes and results ¹. It says review authors may need to do some detective work ¹.

Cochrane's rules

Standard	Status	What it requires
C42 Collating multiple reports	Mandatory ¹	Collate multiple reports so each study, not each report, is the unit. Do not discard secondary reports. Choose and justify which report to use for results ¹
C43 Using data collection forms	Mandatory ¹	Use a piloted data collection form. Cochrane says the form is the historical record of the provenance of the data used ¹
C45 Extracting study characteristics in duplicate	Highly desirable ¹	At least two people working independently ¹
C46 Extracting outcome data in duplicate	Mandatory ¹	At least two people working independently, with a set process for disagreements ¹

Criteria for linking reports

Cochrane says review authors should use as many trial characteristics as possible to link reports ¹. ICMJE asks authors to list a trial's registration number the first time they use the trial's acronym ⁴.

Trial registration numbers ¹.
Authors' names ¹.
Date and duration of the study, length of follow-up, or subgroups selected for secondary goals ¹.
Where doubt remains, correspondence with the study authors or sponsors ¹.

Reporting the process

PRISMA 2020 consists of a statement paper, a checklist, an expanded checklist, an abstract checklist and template flow diagrams for original and updated reviews ². The flow diagram maps the number of records identified, included and excluded, and the reasons for exclusions ⁵. Cochrane says flow diagrams modified from PRISMA can help when collating information from multiple reports ¹.

When reports of one study disagree

Cochrane says a single source rarely gives complete information about a study, and multiple sources may contain conflicting information about the same study ¹. Review authors should say in the protocol how they will handle multiple reports, for example by pre-specifying which report will be used when sources conflict and the conflict cannot be resolved by contacting the investigators ¹.

Where data from all reports go into one form, Cochrane says review authors should identify the main data source for each study when sources conflict ¹. It cites one study that found data extraction errors in 20 out of 34 reviews, which is why it asks for duplicate extraction of outcome data ¹.

Duplicates, misconduct and retractions

Cochrane says plagiarism of results data in the form of duplicated publication may, if undetected, lead to participants being double counted ¹. It says lists of data such as in a meta-analysis can help detect duplicated studies ¹. It advises review authors to examine retraction statements and errata for included studies ¹.

ICMJE defines duplicate publication as publication of a paper that overlaps substantially with one already published, without clear, visible reference to the earlier paper ³. Secondary analyses of a trial should cite the parent trial's registration number and not be registered as separate trials ⁴.

The same logic in news

A review counts studies, not articles. A reader of the news should count sources, not stories. NPR's accuracy guidance calls it a red flag when outlets reporting something all rely on the same single source ⁶.

Common questions

What is the unit of analysis in a systematic review?

The study. Cochrane says studies rather than reports of studies are the principal unit of interest ¹.

How do reviewers spot two reports of the same trial?

Cochrane lists trial registration numbers, authors' names, dates, duration, follow-up and subgroups, and contact with authors where doubt remains ¹.

What happens if a duplicate is missed?

Cochrane says participants may be double counted in a synthesis ¹.

Does PRISMA 2020 include a flow diagram?

Yes. It provides template flow diagrams for new and updated reviews ^2,5.

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Sources

Duplicate publication rules in research
What the ICMJE Recommendations say about duplicate and secondary publication, preprints and registry results, and why it matters for evidence.
Clinical trial registration explained
How ICMJE, WHO, US and EU rules make trials record their plans and results in public registries, with the deadlines, data items and registries that count.
Retractions and corrections in research
What COPE, ICMJE and US research misconduct rules say about corrections, retractions and expressions of concern, and how to check a paper's status.
Circular reporting explained
How one source can look like many, what newsroom guides and US intelligence reviews say about it, and the checks that published standards set out.