What open access means
Open access (OA) means a paper is free to read online. Often it's also free to reuse under a licence such as Creative Commons. You'll see a few labels:
- Gold OA. The publisher makes the final version free on the journal site.
- Green OA. The author puts a version (often the accepted manuscript) in a repository, such as PubMed Central or a university repository.
- Free to read but not licensed for reuse. You can read it, but the reuse terms are narrower. Check the licence.
- Preprints. These are free versions posted before peer review. They're useful but not peer reviewed, so label them clearly.
"Free abstract" is not open access. If only the abstract is open, treat it as abstract-only evidence and say so.
Why OA matters for students
- You can read the whole paper. Risk-of-bias judgements need methods sections, not abstracts.
- Your supervisor and examiners can check your sources without hitting a paywall.
- Access doesn't vanish when your login does. That matters if you'll update the review or publish it later.
- Copyright is clearer. Licensed OA tells you what you can quote, adapt or share.
Where to search, and what each source is for
PubMed and PMC. PubMed indexes biomedical literature. PMC holds full text. To limit to the PMC Open Access Subset, use the filter open access[filter] in PMC, or pubmed pmc open access[filter] in PubMed. Licence terms still vary by article, so check each one.
Europe PMC. A large life-sciences database run in Europe. Search it alongside PubMed, and record it as a separate source with its own date.
DOAJ (Directory of Open Access Journals). A curated index of open-access journals. Use it to check an unfamiliar journal before you lean on it.
Trial registries. ClinicalTrials.gov, the WHO International Clinical Trials Registry Platform (ICTRP) and the ISRCTN registry are free to search. Use them to find ongoing or unpublished trials and to compare planned outcomes with published ones. A registry record isn't a publication. Report it as a registry record.
Unpaywall. An open database of free scholarly articles harvested from publishers and repositories. Use it to check whether a free, legal version of a paper exists.
Preprint servers such as medRxiv, a free server for health-sciences preprints. Preprints haven't been peer reviewed. Include them only if your protocol allows it. Flag them as preprints and check later whether a peer-reviewed version came out.
A simple OA search routine
- Write your PICO. See how to write a PICO question.
- Build one search string with OR inside each concept and AND between concepts.
- Run it in PubMed with the OA filter, then in Europe PMC. Save the exact string and the date for each.
- Search two registries for your intervention and condition.
- Check unfamiliar journals in DOAJ. Check closed papers with Unpaywall for a legal free copy.
- Log OA status for every included study: full text open, abstract only, or registry record. Your PRISMA methods section will need the sources and dates. See the PRISMA 2020 guide.
Limiting to OA can introduce bias, because some relevant studies may be closed. Say that you limited to OA, and discuss it as a limitation. Examiners respect a stated limit far more than a hidden one.
How Systematic Review handles this
Systematic Review works from open-access sources only. It doesn't surface paywalled full text as if it were open. A search path in the app is Coming soon. For now, run your main search with the routine above.
FAQ
Is a free abstract the same as open access?
No. OA means the full text is free to read. Abstract-only evidence should be labelled as such.
Will an OA-only search be accepted for my dissertation?
Often, yes, if you state it and discuss it as a limitation. Check with your supervisor before you register your protocol.
Are preprints OK in a systematic review?
They can be, if your protocol allows them. Label them and check for a peer-reviewed version.
What's the PubMed open-access filter?
pubmed pmc open access[filter] limits PubMed results to articles in the PMC Open Access Subset.
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