How to Write a Peer Review: A Reviewer Guide

Peer review is one of the most consequential acts of scientific citizenship a researcher can perform, yet few graduate programs provide systematic training in how to do it well. This guide addresses that gap directly. It covers what a competent peer review actually contains, how to structure your assessment, where reviewers commonly fail, and what professional standards govern the process.

What You Are Being Asked to Do

When a journal invites you to review a manuscript, you are not being asked to decide whether you would have conducted the study differently. You are being asked to evaluate whether the work is scientifically sound, whether the conclusions are justified by the evidence presented, and whether the manuscript meets the standards of the field and the journal. Those are three distinct questions, and conflating them is among the most common sources of reviewer error.

The Committee on Publication Ethics (COPE), whose guidelines govern peer review conduct at thousands of journals worldwide, defines the reviewer's core obligation as providing "a fair, honest, and timely review of the scientific work." COPE's Ethical Guidelines for Peer Reviewers (available at publicationethics.org) make clear that reviewers serve the scientific record, not authors, not editors, and not their own disciplinary preferences.

The International Committee of Medical Journal Editors (ICMJE), whose Recommendations for the Conduct, Reporting, Editing, and Publication of Scholarly Work in Medical Journals are adopted by over 5,000 journals, adds a confidentiality obligation: the manuscript is a privileged communication that cannot be shared, cited, or discussed without the editor's permission.

Understanding these obligations before you begin is not a formality. It shapes every decision you make during the review.

Reading the Manuscript Before You Write Anything

A competent review begins with a reading protocol, not a writing protocol. Read the manuscript at least twice before drafting a single comment.

On the first pass, read straight through without annotation. Your goal is to understand what the authors claim to have done and what they claim to have found. At the end of the first reading, you should be able to state the central hypothesis, the primary methodology, and the main conclusion in two or three sentences. If you cannot, that is itself a significant finding about the manuscript's clarity.

On the second pass, read critically and annotate. Mark claims that lack citation, methods that are underspecified, statistical choices that need justification, figures that do not support the text, and conclusions that exceed what the data demonstrate. Note the study design relative to what the research question actually requires. A cross-sectional observational study cannot establish causation regardless of how confidently the authors frame their results.

Pay particular attention to the methods section. The Council of Science Editors (CSE), which sets standards for scientific publication across the natural sciences, emphasizes that methods must be reported with sufficient detail to permit replication. If you cannot determine from the methods section how the study could be reproduced, that is a substantive deficiency, not a stylistic preference.

For context on how peer review fits into the broader publication process, see How It Works and Key Dimensions and Scopes of Peer Review.

Structuring Your Written Assessment

A well-structured peer review contains four components: a summary, major concerns, minor concerns, and a recommendation. Each has a distinct function.

The summary (two to four sentences) demonstrates to the editor and authors that you read the manuscript carefully enough to understand it. It is not a compliment and not a verdict. It is evidence of comprehension.

Major concerns are issues that, if unresolved, would prevent the manuscript from being published. These include fundamental methodological flaws, conclusions not supported by data, missing controls that are required for the claims being made, statistical errors that affect interpretation, and ethical issues such as missing IRB approval or undisclosed conflicts of interest. Each major concern should be described specifically enough that the authors can respond to it. "The statistics are inadequate" is not a major concern — it is a complaint. "The authors apply a paired t-test to data that violate the normality assumption (Shapiro-Wilk p < 0.05, as shown in their own supplementary Figure S3) without acknowledging this or applying a nonparametric alternative" is a major concern.

Minor concerns are issues that should be corrected but do not affect the fundamental validity of the work: ambiguous terminology, missing units, figure labels that do not match text references, grammatical errors that impede comprehension, and citations to retracted or superseded literature.

The recommendation — accept, minor revision, major revision, or reject — should follow logically from your concerns. If you have identified no major concerns, a rejection recommendation requires explicit justification. If you have identified multiple fatal flaws, a minor revision recommendation is incoherent.

Tone, Specificity, and What Editors Actually Need

Editors use peer reviews to make decisions and to communicate substantive feedback to authors. A review that says "this paper needs more work" provides neither. A review that identifies specific problems, explains why they are problems, and in some cases suggests how they might be addressed serves both functions.

Tone is a professional matter, not merely an ethical one. Aggressive, dismissive, or sarcastic language in a peer review is grounds for an editor to discard the review entirely, and under COPE guidelines, it constitutes a breach of reviewer conduct. Criticism of the work is appropriate and necessary. Criticism of the authors is not.

Do not review manuscripts outside your expertise. If you are asked to evaluate a manuscript whose statistical methods, domain knowledge, or specialized techniques fall outside your competence, inform the editor immediately. Accepting a review and then making uninformed judgments about methods you do not understand causes direct harm to authors and to the scientific record. The Peer Review FAQ addresses common questions about declining invitations and managing conflicts of interest.

Conflicts of Interest and Confidentiality

You must decline a review invitation if you have a financial interest in the outcome, a competitive relationship with the authors, a close collaborative or personal relationship, or prior knowledge of the manuscript through any channel. These are not judgment calls. They are hard boundaries under both COPE guidelines and the policies of the National Institutes of Health, which requires conflict-of-interest disclosure in its Scientific Review process under 42 U.S.C. § 289a.

The manuscript you are reviewing is confidential until it is published. You may not use its data, methods, or ideas in your own work before publication. You may not discuss its contents with colleagues. You may not share it with a trainee to help conduct the review without explicit editor permission and disclosure of that trainee's identity.

When to Seek Additional Guidance

Peer review involves judgment calls that this guide cannot fully anticipate. If you encounter suspected data fabrication or image manipulation during your review, do not simply reject the manuscript — report your concern directly to the editor with specific documentation. COPE provides a case repository and flowcharts for handling suspected misconduct (publicationethics.org/resources/flowcharts).

If you are navigating a dispute about a review you have submitted, or seeking recourse as an author who believes a review was conducted in bad faith, see How to Get Help for Peer Review for guidance on escalation pathways and relevant professional bodies.

The Peer Review FAQ covers additional procedural questions, including response timelines, anonymity models, and post-publication review. For a broader orientation to the peer review landscape, the site index provides a full map of available resources.

References: Committee on Publication Ethics (COPE), Ethical Guidelines for Peer Reviewers, publicationethics.org; International Committee of Medical Journal Editors, Recommendations for the Conduct, Reporting, Editing, and Publication of Scholarly Work in Medical Journals, icmje.org; Council of Science Editors, CSE's White Paper on Promoting Integrity in Scientific Journal Publications, councilscienceeditors.org; National Institutes of Health, NIH Grants Policy Statement, 42 U.S.C. § 289a.

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