Supplementary Materials: What to Include & Format

Supplementary materials — also called supporting information, depending on the publisher — are files an author submits alongside a manuscript that are not part of the typeset article itself but are made available with it: extended datasets, additional figures or tables, methods detail, code, videos, or other content that supports the paper’s conclusions without fitting inside the journal’s length or format constraints for the main text.

They are a routine part of journal submission across most disciplines, but authors consistently misjudge two things about them: how much editorial scrutiny they receive, and what actually belongs in them versus in the main article. Both misjudgments create real problems at review and after publication — reviewers flag manuscripts that lean on unreviewed-feeling supplementary content to reach their conclusions, and readers cannot evaluate a paper’s central claims if load-bearing evidence is buried in a file most people never open.

What counts as supplementary material

Publishers use overlapping but not identical terms for this category: Elsevier and Nature call it “supplementary material” or “supplementary information,” PLOS calls it “supporting information,” and some journals distinguish “supplementary data” (datasets) from “supplementary methods” or “supplementary figures/tables.” The underlying concept is the same across all of them: content submitted with the manuscript, reviewed alongside it, and published as a companion to — not a substitute for — the main article.

Typical categories include:

Extended or raw data — datasets too large or granular for the main text, often as CSV, Excel, or plain-text files.
Additional figures and tables — supporting visualizations that are informative but not essential to following the main argument (e.g., a full set of dose-response curves when the main text shows only the representative case).
Extended methods — detailed protocols, reagent lists, instrument settings, or statistical code that would clutter the Methods section but are needed for someone to replicate the work.
Multimedia — video, audio, or animation that cannot appear in a print-formatted article.
Questionnaires, survey instruments, or interview guides — the actual materials used to collect data, as distinct from the results derived from them.

Elsevier’s guidance describes this category as content offering “additional possibilities to publish supporting applications, movies, animation sequences, high-resolution images, background datasets, sound clips and more” (Elsevier Journal Article Publishing Support Center). It is explicit, though, that requirements differ by journal and that authors should always check the specific journal’s Guide for Authors rather than assume one house style applies everywhere on the platform.

The scope rule most publishers converge on

Wording varies, but the great majority of publisher policies converge on the same two-part scope rule:

The main article must stand alone. A reader who never opens the supplementary files should still be able to follow the argument and understand the basis for the paper’s conclusions from the main text alone.
Supplementary material must directly support, not duplicate, the main conclusions. It should be “additional or complementary and directly relevant to the article content” — material that could not fit in the printed/typeset article for reasons of length or format, not a parking lot for content an author didn’t want to cut during revision.

SAGE frames this as material “used to create the research (such as datasets) or additional pieces to the article that could not be included in the print version” (SAGE supplemental material guidelines). Nature’s authors’ page similarly describes Supplementary Information as material “directly relevant to the conclusion of a paper” that cannot be included in the main text “for reasons of space or medium” — language that also makes explicit the point covered next: that it is peer-reviewed.

In practice, editors and reviewers apply this scope test by asking whether a reader who skipped the supplement entirely would still trust the paper’s central claims, and whether anything in the supplement is quietly doing work the main text implies has already been done. If a result the abstract or Discussion relies on only appears in a supplementary table, that is usually a sign the material belongs in the main text, not the supplement — see CASRAI’s guide on manuscript word count limits for how to decide what to cut versus what to move when a submission is over length.

Common misconception: supplementary material is not “unreviewed”

A persistent misconception among first-time and even experienced authors is that supplementary files sit outside peer review — that because they aren’t typeset or copyedited, they also aren’t scrutinized. This is generally false. At most major publishers, supplementary material is reviewed as part of the manuscript, by the same reviewers, as part of the same accept/reject decision:

SAGE states plainly that “supplemental material files are subjected to peer-review alongside the main article” and that decisions about whether to include them are made by the editorial team as part of the acceptance process.
Nature describes its Supplementary Information as “peer-reviewed material” in the same breath as defining what belongs in it.
PLOS ONE’s Supporting Information guidelines direct authors to match every supporting-information caption to a file the reviewers can open — a requirement that only makes sense if reviewers are expected to actually examine the files (PLOS ONE Supporting Information guidelines).

What differs from the main text is copyediting and typesetting, not review: supplementary files are generally published “as provided” by the author, without the layout and language polish applied to the main article. That production-stage difference is where the “unreviewed” misconception seems to originate, but it is a claim about formatting, not about scrutiny. Authors should draft and check supplementary material with the same care as the main manuscript — errors, inconsistent labeling, or unsupported claims in a supplementary table are a legitimate basis for a reviewer to raise concerns or for an editor to request revision, exactly as they would be if the same content appeared in the main text.

Some journals do vary in how much reviewers are formally required to evaluate supplementary content versus simply given the option to consult it, so authors should not assume every reviewer scrutinizes every supplementary file line-by-line — but “not scrutinized at all” is the wrong default assumption, and is not what the major publisher policies describe.

Common format requirements

Exact specifications vary by journal, but recurring conventions across publishers include:

File types

Most publishing platforms accept a broad range of formats — documents (PDF, DOCX), datasets (CSV, XLSX), images, audio, and video — but several publishers actively recommend against submitting data as PDF when it will be reused: Elsevier’s guidance specifically recommends text, Word, or Excel files over PDF for supplemental tables and mechanisms “to aid use by other authors,” since PDF tables are hard to extract data from. Journal-specific Guides for Authors are the authoritative source for accepted formats and size limits; SAGE recommends keeping files under 10 MB where possible, and Elsevier sets a preferred maximum of 150 MB per file (1 GB in total) — figures that illustrate the range across publishers rather than a fixed universal cap.

Naming conventions

Two common but different conventions exist:

Sequential, type-labeled naming (Elsevier and similar): files named by type and number as submitted — “Fig. S1,” “Table S1,” and so on — with the file name matching the content it contains.
“S-number” item naming (PLOS): every supporting information item gets an identifier combining “S” and a whole number — “S1 Table,” “S1 Appendix,” “S2 Fig” — with related sub-parts combined into a single file rather than split as S1A/S1B.

In both systems, the operative rule is the same: the file name and the in-text caption/citation must match exactly, since mismatches are a common cause of production delays and reader confusion after publication.

Citing supplementary items in text

Supplementary items should be cited explicitly at the point in the main text where they are relevant — “see Supplementary Table S1” or “full dose-response curves are provided in Fig. S2” — rather than referenced only in a blanket statement at the end of a section. PLOS’s own guidance specifically warns authors to phrase nested references carefully (for example, “Table A in S1 Text” rather than “S1 Table in S1 Text”) to avoid broken cross-reference links in the published version. Authors should also avoid using tracked changes or annotation layers in submitted supplementary Office files, since publishers generally post these files exactly as submitted — visible tracked changes will appear in the published version.

Deciding what belongs in supplementary material versus the main text

There is no universal checklist because journal length limits, disciplinary norms, and article type all shift the line, but the following questions are a reasonable working test, consistent with how the publisher guidance above frames the scope rule:

Is it necessary to trust the main conclusion? If removing it would leave a reader unable to evaluate whether the paper’s central claim is supported, it belongs in the main text — see also CASRAI’s guidance on writing a data availability statement, which covers the related but distinct question of where underlying research data should be deposited versus described.
Is it needed to replicate the method, but not to follow the argument? Detailed protocols, full statistical code, and exhaustive parameter lists are strong supplementary-material candidates — necessary for reproducibility, not necessary for a first read.
Is it a robustness check or secondary analysis? Sensitivity analyses, additional cohorts, or alternative model specifications that corroborate but don’t drive the main finding typically belong in the supplement.
Would it exceed a journal’s word, figure, or table limit if included in the main text? This is often the practical trigger for moving otherwise-central content to the supplement — but per the scope rule above, this should be a last resort, not a first move, precisely because material load-bearing for the conclusions is supposed to stay in the main text regardless of length pressure.
Is it raw data rather than a dataset in its own right? Large raw datasets are frequently better placed in a dedicated data repository with a data availability statement pointing to it, rather than as a supplementary file — repositories are built for discoverability, citation, and long-term preservation in ways an unindexed supplementary file typically is not. CASRAI’s guide on choosing an open data repository covers that decision in more depth.

When in doubt, editors generally prefer material to be excluded or moved to a repository rather than added defensively “just in case” — bloated supplements slow review and dilute the material that actually needs a reviewer’s attention.

How this differs from related submission elements

Supplementary material is often confused with, but distinct from, a few adjacent submission requirements:

A graphical abstract is a single summary visual published with the abstract, not additional evidentiary content.
A Highlights bullet-point summary is a short promotional/indexing element, not supporting data.
A data availability statement tells readers where underlying data can be found (often in a repository); the supplementary file itself is one possible location, but not the only or always the best one.
Materials submitted “for reviewer/editor eyes only” during review (e.g., a manuscript tracked-changes file, or confidential supporting data) are a different category from published supplementary material and are typically marked accordingly during submission through systems like ScholarOne or Editorial Manager.

Frequently asked questions

Is supplementary material peer reviewed?

At most major publishers, yes — it is reviewed as part of the manuscript and factors into the accept/reject decision. What it typically does not receive is copyediting or typesetting; it is generally published as submitted. Always check the specific journal’s policy, since the degree of formal reviewer scrutiny of supplementary content can vary.

Can I add supplementary material after a manuscript is accepted?

Generally no, or only with editorial approval. Because supplementary material is reviewed alongside the main manuscript, adding new supplementary content post-acceptance would mean it never went through that review — most journals restrict this and require any changes to go through the editorial office rather than being added silently at proof stage.

Should raw data go in supplementary material or a repository?

Increasingly, journals and funders prefer a dedicated data repository (with a data availability statement in the main text linking to it) over supplementary files for large or reusable datasets, since repositories support persistent identifiers, structured metadata, and long-term access in ways a static supplementary file attached to a PDF typically does not.

Do supplementary files count toward a journal’s word or page limit?

Usually not toward the main-text word count, but many journals cap the number or size of supplementary files, and some require a separate short summary of supplementary content within the main text. Check the specific journal’s Guide for Authors — this is exactly the kind of journal-specific variation that makes “just move it to supplementary” an unreliable substitute for actually trimming the main manuscript when it is over length.

What file format should I use for supplementary tables?

Where the journal allows a choice, an editable format (Word, Excel, CSV) is generally preferable to PDF for tabular or numeric supplementary content, since it lets other researchers reuse the data — a preference Elsevier states explicitly in its author guidance. PDF remains appropriate for supplementary content that is inherently document-like, such as a full survey instrument or extended protocol.

Key takeaways

Supplementary material (or supporting information) supports a manuscript’s conclusions without being part of the typeset article — it is not a place to relocate content just because a manuscript runs long.
The main article must stand alone; supplementary content should directly support, not duplicate, the main conclusions.
Supplementary material is typically peer reviewed alongside the main manuscript at major publishers — the common belief that it escapes review is a misconception, though it is generally not copyedited or typeset.
File type, naming convention, and size limits vary by publisher — always confirm against the specific journal’s Guide for Authors rather than assuming a single fixed standard.
Large reusable datasets are often better placed in a dedicated repository with a data availability statement than as a static supplementary file.