What Is Peer Review, and How Can You Tell If an Article Is Peer Reviewed?

Teachers and editors often ask for "peer-reviewed sources". Peer review is the quality check that separates a journal article from a blog post or a preprint, and with a little practice you can tell in a minute whether a paper has been through it.

What peer review is

When researchers submit a paper to a journal, the editor sends it to two or three other experts in the same field. These reviewers read it critically. They check whether the methods are sound, whether the conclusions follow from the evidence and whether earlier work has been taken into account. They recommend that the paper be accepted, revised or rejected. Most papers are revised at least once before they are published.

There are a few variants. In single-blind review the reviewers know who the authors are but stay anonymous themselves. In double-blind review neither side knows the other's identity. In open review the reports, and sometimes the reviewers' names, are published with the paper.

Five ways to check

• Look at the journal's website. A peer-reviewed journal says so, usually on a page called "About the journal" or "Peer review policy".
• Look for dates on the article. Reviewed papers normally show when they were received, revised and accepted. A gap of weeks or months between those dates is a sign that review took place.
• Check what kind of article it is. Journals also publish editorials, news, letters and book reviews, which are not peer reviewed even in a peer-reviewed journal. Research articles and review articles are.
• Check where it is hosted. Papers on arXiv, bioRxiv, medRxiv and SSRN are preprints and have not been reviewed, although many go on to be published. See preprint versus published paper.
• Use a library filter. Library search tools and many databases have a "peer-reviewed" tick box that limits results to reviewed journals.

Checking that the journal is genuine

Some journals claim to review papers but publish almost anything for a fee. They are often called predatory journals. Warning signs include promises of publication within days, a very broad subject range and emails soliciting papers. Good signs are that the journal is indexed in a recognised database such as PubMed, Scopus or Web of Science, or, for an open-access journal, listed in the Directory of Open Access Journals.

Has the paper been retracted?

Peer review reduces errors but does not remove them. Papers are sometimes corrected or withdrawn after publication. To check:

• Look at the article's page on the publisher's site, where a retraction or correction notice is displayed prominently.
• Look for a Crossmark button on the page or the PDF. Clicking it shows whether the paper has been updated or retracted.
• Search the Retraction Watch database, which records retracted papers across all publishers.

What peer review does not tell you

A reviewed paper is not guaranteed to be right. Reviewers do not usually repeat the experiments, and a single study can be overturned by later work. Treat peer review as a minimum standard, then read the paper with the questions in our guide to how to read a research paper.

Reading the peer-reviewed version for free

The free copy of a paper is often the accepted manuscript, which is the text after peer review. Our finder tells you which version each free copy is, so you know whether you are reading the reviewed text or an earlier preprint.