TL;DR

• Supplementary material holds data, figures, or methods too detailed for the main paper.

• Most peer-reviewed journals accept and encourage supplementary files.

• Reviewers can access it, but it rarely affects the core argument alone.

• Label every item clearly so readers can find it without confusion.

• Not all journals handle supplementary files the same way.

You have finished your research. The paper is written. Then you realise your results section is thirty pages long, your raw data sits in a spreadsheet no one has seen, and your methods section reads like a textbook chapter. Something has to give. Most journals impose strict word limits, and editors will not bend them for a first-time author.

This is exactly the problem a supplementary material section solves. It gives your research room to breathe without inflating the main manuscript. Understanding what goes there, what stays in the body, and how journals treat these files can save you a rejection and a lot of confusion.

The distinction between essential and supplementary content is one of the most practical judgements a researcher learns to make. Everything below will help you make it correctly.

What Is a Supplementary Material Section

A supplementary material section is a collection of files, figures, tables, or datasets that support a published paper but sit outside the main manuscript. Journals host these files alongside the article. Readers can download them, but the paper's core argument must stand without them. Think of supplementary material as evidence that strengthens your work without cluttering the narrative.

Journals began formalising supplementary material policies in the late 1990s as digital publishing made it practical to host large files online. Today, major publishers including Springer Nature, Elsevier, and Wiley each publish explicit author guidelines describing what they will and will not accept as supplementary content. The formats vary, but the logic is consistent: the main paper tells the story, and the supplementary section provides the raw material behind it.

Common items that appear in a supplementary material section include extended data tables, additional figures that support but do not drive the argument, full statistical outputs, survey instruments, interview transcripts, video files, code repositories, and detailed protocol descriptions. If a reader needs it to replicate your study but does not need it to understand your conclusion, it likely belongs in supplementary material.

One thing worth understanding early: supplementary material is peer-reviewed. Reviewers receive access to it alongside the main manuscript. This means it must be accurate, clearly labelled, and professionally presented. Sloppy supplementary files can and do contribute to rejection decisions.

What Should Go in a Supplementary Material Section

Supplementary material should contain content that is real, relevant, and reproducible, but too granular for the main text. Raw datasets, extended methods, and additional validation figures are the most common examples. If removing an item from the main paper would make it shorter without making it less convincing, that item is a candidate for supplementary placement.

A useful way to think about this is to ask three questions about each element in your draft. First, does the reader need this to follow the argument? If yes, keep it in the body. Second, does the reader need this to reproduce the study? If yes, it belongs somewhere in the paper, and supplementary material is often the right place. Third, is this detail that only a specialist would pursue? If yes, supplementary material is almost certainly correct.

Here is a practical sequence for deciding what to move:

1. List every figure, table, and appendix in your current draft.

2. For each one, write one sentence explaining why a general reader needs it in the main text.

3. If you cannot write that sentence convincingly, flag the item for supplementary placement.

4. Check your target journal's author guidelines to confirm what file types and sizes they accept.

5. Compile the flagged items into a single supplementary document or separate files, depending on the journal's instructions.

If you are still learning how to structure the sections that stay in the main manuscript, the guide on how to write a methodology section for a science paper covers what level of detail belongs in the body versus what can reasonably move to supplementary files.

How Journals Handle Supplementary Material Differently

Journal policies on supplementary material are not uniform. Some journals host files on their own servers indefinitely. Others link to third-party repositories. A small number of journals do not accept supplementary material at all and require authors to deposit data in public repositories like Zenodo or Figshare before submission. Checking the specific policy of your target journal before you format your files is not optional.

For example, journals published under the Public Library of Science (PLOS) framework require that all data underlying the findings be made fully available without restriction, either in the supplementary files or in a public repository. Their data availability policy is stated explicitly in their author guidelines. The journal Nature and its family of journals have their own extended data format, which is distinct from supplementary information and appears in a specific section of the published article with its own formatting rules.

Student researchers submitting to journals that publish high school and undergraduate work, such as the Columbia Junior Science Journal or the International Journal of High School Research, should read each journal's submission page carefully. Policies at student-focused journals can differ significantly from those at major commercial publishers.

If you are joining the waitlist for Publication Compass, the platform helps you match your paper to journals whose formatting requirements fit your manuscript, including how they handle supplementary files.

How to Format a Supplementary Material Section Correctly

Formatting supplementary material correctly means making it navigable for a reviewer who may spend limited time on it. Every item needs a clear label, a short title, and a reference to the point in the main text where it is cited. Reviewers should never have to hunt for what a supplementary figure is showing or why it exists.

Follow this sequence when preparing your supplementary files:

1. Number every item in order of citation in the main text. Use labels like Figure S1, Table S2, or Appendix S1, where the S signals supplementary.

2. Write a short caption for every figure and table. The caption should be self-contained. A reader who has not read the main paper should understand what the item shows.

3. Cite each supplementary item in the main text at the first point where it becomes relevant. Write something like: full results are provided in Table S1.

4. Compile all supplementary items into the format your journal requests. Some journals want a single PDF. Others want separate files. Some want a supplementary methods document kept separate from supplementary data.

5. Include a brief legend or table of contents at the start of your supplementary document listing every item and its page or file location.

One formatting mistake that appears frequently in student submissions is placing critical information only in supplementary material. If a finding is central to your conclusion, it must appear in the main text. Reviewers and editors are alert to this. Hiding important results in supplementary files to avoid word count limits is considered poor practice and can raise questions about transparency.

Understanding how peer review works will help you see why this matters. Reviewers evaluate whether your argument is complete and honest. The guide on what peer review is and what happens to your paper explains the full process from submission to decision.

Common Mistakes Students Make with Supplementary Material

The most common mistake is treating supplementary material as a place to put everything that did not fit. Supplementary files should be curated, not a dump of leftover content. A reviewer who opens a supplementary document and finds fifty pages of unorganised tables and unlabelled figures will not form a positive impression of the paper.

A second frequent error is failing to cite supplementary items in the main text. If a figure or table in your supplementary section is never referenced in the body of the paper, it raises an immediate question: why is it there? Every supplementary item must be called out explicitly in the main manuscript.

A third mistake involves file formats. Journals often specify accepted formats. Submitting a supplementary video in a format the journal cannot host, or a data file in a proprietary format that reviewers cannot open, creates unnecessary friction. Always check the technical requirements before you submit.

Finally, some students omit supplementary material entirely when it would strengthen their paper significantly. If you conducted a survey, the instrument belongs somewhere readers can access it. If you wrote code to analyse your data, making it available adds credibility. Transparency in research is increasingly expected, not just encouraged. The Committee on Publication Ethics (COPE), a recognised body that sets standards for academic publishing, emphasises data transparency as a core principle of responsible research.

When You Do Not Need a Supplementary Material Section

Not every paper needs supplementary material. Short papers, review articles, and opinion pieces rarely include it. If your main manuscript already contains everything a reader needs to understand and evaluate your work, and the word count is within the journal's limit, there is no obligation to create supplementary files.

Adding supplementary material unnecessarily can actually work against you. It signals to reviewers that you may not have made clear decisions about what matters in your research. A focused, well-structured paper with no supplementary material is often stronger than a sprawling one where important content has been offloaded to appendices.

Before you decide to include supplementary material, ask whether each item genuinely adds value for a reader who wants to go deeper. If the honest answer is no, leave it out.

Frequently Asked Questions

What is a supplementary material section in simple terms

A supplementary material section is extra content attached to a published paper, such as raw data, additional figures, or detailed methods. It supports the main paper but is not essential for understanding the core argument. Readers can download it, but the paper must make sense without it.

Do peer reviewers read supplementary material

Yes. Peer reviewers receive access to supplementary files alongside the main manuscript. They are expected to assess whether the supplementary content is accurate and relevant. Poorly prepared supplementary material can contribute to a negative review decision, so it should be treated with the same care as the main paper.

Can I put my raw data in the supplementary material section

Yes, raw data is one of the most appropriate types of content for supplementary material. Many journals actively encourage or require it. Some journals ask authors to deposit data in a public repository like Zenodo or Figshare and then link to it from the paper, rather than attaching files directly.

What happens if I forget to cite a supplementary figure in my paper

An uncited supplementary figure is a red flag for reviewers. It suggests the content may be unnecessary or that the paper is disorganised. Every item in your supplementary section must be referenced at least once in the main text, at the point where it becomes relevant to the reader.

Is supplementary material the same as an appendix

They serve a similar purpose but are not identical. An appendix typically appears at the end of the main document and is part of the manuscript. Supplementary material is a separate file or set of files hosted alongside the published article. Some journals use the terms interchangeably, so always check the specific journal's guidelines.

Getting Your Paper Ready to Submit

Supplementary material is a tool. Used well, it makes your paper cleaner and your research more transparent. Used poorly, it creates confusion and signals that the manuscript is not ready. The decision about what belongs in the main text and what belongs in supplementary files is one of the clearest tests of whether a researcher understands their own work.

Start with your target journal's author guidelines. Build your supplementary section from there, labelling every item, citing each one in the body, and formatting files in the required format. If you want structured guidance on matching your paper to the right journal and preparing your submission correctly, explore the resources at the Publication Compass blog.