A well-run paper is disciplined about what it shows. It presents exactly what the reader needs to follow the argument, and no more. Everything else that supports the work but would clog the narrative — extended methods, extra figures, large tables, raw outputs — goes into supplementary materials. Used well, the supplement keeps your paper readable while making your work more complete. Used badly, it becomes a place to hide things reviewers should have seen.
What supplements are for
Supplementary materials are the right home for content that is useful but not essential to the main story:
- Extended methods or protocols that would interrupt the flow.
- Additional figures and large data tables.
- Videos, audio, or interactive content that print cannot carry.
- Full survey instruments, checklists, or code.
What must stay in the paper
Here is the line that trips authors up: anything a reader needs to understand or trust your conclusions belongs in the main text. Your key results, the analyses that support your claims, and the figures that carry your argument cannot be demoted to the supplement to hit a word limit. Reviewers may not read supplements closely, and readers certainly skim them — so burying a crucial result there weakens your paper and invites the suspicion that you were hiding it.
Test: if a reader read only the main paper, could they judge whether your conclusions are justified? If not, something has been misfiled in the supplement.
Format and cite them properly
Give supplementary items a clear numbering scheme (Supplementary Figure S1, Supplementary Table S1) and refer to each one in the main text, in order. Provide them in open, accessible formats — PDF for documents, standard formats for data — not proprietary files a reader may not be able to open. Each supplementary figure and table still needs a caption that stands alone.
Make large data findable, not loose
A big dataset attached as a spreadsheet is hard to find and easy to lose. For anything substantial, deposit it in a repository following the FAIR principles, cite it with its DOI, and let the supplement point to it. That way your data is preserved, citable and reusable — which a file bolted onto a PDF never is.