01 · The Question
Where Is the Appendix or Protocol the Paper Keeps Referring To?
You are reading a paper and encounter a sentence such as “full eligibility criteria are provided in Appendix 2,” “the complete procedure is described in the study protocol,” or “additional methodological details are available online.” You look at the end of the PDF. Nothing. You inspect the publisher page. Still nothing obvious.
This can become consequential when the missing document contains information needed to understand how the study was designed, conducted, or analyzed. Published studies do sometimes place protocols and appendices in supplementary files or external records. For example, contemporary journal articles may explicitly identify a full trial protocol as a supplementary appendix, while others point readers to a separately registered protocol.
Before concluding that the material is unavailable, determine exactly what document the authors meant, where it was originally supposed to reside, and whether it can be traced through the scholarly record.
03 · What You Need to Know
First Work Out What Kind of Document You Are Looking For
“Appendix” can refer to several different things
The terminology is less standardized than readers sometimes expect. An appendix may be embedded at the end of the article, supplied as a separate supplementary file, incorporated into an online-only version, or hosted elsewhere. A protocol can similarly appear as supplementary material, a separately published article, a registry record, or a repository object.
Appendix
Additional material associated with the paper, which may be embedded in the article or supplied separately.
Study protocol
A document describing how a study is planned or conducted. Depending on the study and publication practice, it may appear with the article, in a registry or repository, or as a separate publication.
These possibilities matter because searching only the article PDF can produce a false negative. A published example in Scientific Reports, for instance, explicitly states that a trial protocol is provided in a supplementary appendix. Another systematic-review article points both to a supplementary protocol and to a separately accessible PROSPERO registration. The location is therefore something to establish, not assume.
Read the paper's reference to the missing material literally
Return to every place where the document is mentioned. The wording may reveal whether you should be looking for “Appendix A,” “Supplementary Appendix 1,” “Online Appendix,” “Study Protocol,” a registry identifier, or a separately cited publication.
Look particularly in the Methods, Data Availability, Supplementary Information, Trial Registration, Protocol and Registration, footnotes, references, and end matter. Record any identifier exactly. A DOI, registration number, accession number, repository identifier, or formal citation is far more useful than searching the web for a generic phrase such as “Smith study protocol.”
Check the current publisher page, not only the PDF
The PDF is one representation of the article. Associated material may be exposed only through the article's web landing page. Publisher pages can list supplementary files separately, sometimes in formats such as DOCX, XLSX, PDF, video, or data files.
Start with the DOI or official article page and look for headings such as Supplementary Information, Supporting Information, Electronic Supplementary Material, Appendix, Associated Data, or similar wording. If the file is listed but inaccessible, you are dealing with a different problem: the supplementary material exists but has its own access barrier.
Determine whether the protocol was published separately
A protocol does not necessarily belong to the same publication object as the final study report. Search the reference list for a protocol paper. Then search using combinations of the study title, acronym, investigators, intervention or topic, and the word protocol.
When you find a candidate, verify that it actually belongs to the study. Compare authors, study population, intervention or exposure, setting, recruitment dates, registration identifiers, and other distinctive characteristics. Similar titles alone are not enough.
Follow registration identifiers
For some study designs, the paper may provide a registration number that leads to additional methodological information. Clinical trials may point to a trial registry, while systematic reviews may identify a registry such as PROSPERO. Other disciplines use different repositories and registration systems.
A registration record is not automatically equivalent to a complete protocol. It may contain only selected prespecified information. Conversely, it may link onward to documents or amendments that help reconstruct the study's methodological history.
Watch Out
Do not silently substitute a registry entry, conference abstract, later paper, or similarly titled protocol for the appendix or protocol cited by the authors. Establish the relationship between the documents before treating them as equivalent.
Check other legitimate versions of the article
An institutional repository, subject repository, or accepted manuscript may contain references or attachments that are easier to locate than those on the publisher platform. However, accompanying files do not necessarily travel with every version of a manuscript.
If you locate another version, compare it with the publisher's record and verify whether the appendix is actually the same object. The distinction between an accepted manuscript and the final published version can matter when links, numbering, supplementary references, or post-acceptance changes differ.
Search by the document itself, not only by the article
If the paper gives the appendix or protocol a title, search that exact title. If it supplies a citation, search the cited work. If it provides a DOI, resolve the DOI. If it provides a registration or accession number, search the relevant registry directly.
This sounds elementary, but missing-document searches often go astray because researchers repeatedly search the article title instead of the identifier belonging to the missing object.
Contact the authors when the document still cannot be found
The corresponding author is an appropriate person to contact when a paper explicitly refers to material that you cannot locate. Give the citation or DOI and name the exact document, such as “Supplementary Appendix 2” or “the prespecified protocol referred to in the Methods.”
Ask whether the document remains available and, if so, where it can legitimately be obtained. A specific request is easier to answer than a general request for “more methodological details.” The practical principles are similar to those involved when you request research material directly from an author.
Contact the publisher when the article appears to promise a missing file
If the current publisher page says that an appendix or supplementary document accompanies the article but no working file appears, contact the journal or publisher. Website migrations, broken links, incomplete file transfers, and metadata problems can separate an article from associated material without meaning that the material never existed.
Save the relevant citation and, when useful, the wording that identifies the missing item. That gives support staff enough information to investigate the record.
The missing protocol can affect how confidently you evaluate the study
Protocols can be important for examining prespecified outcomes, eligibility criteria, analytical decisions, amendments, and other aspects of study design. If the published paper provides all information necessary for your immediate purpose, the missing protocol may not prevent you from using the paper. If your appraisal specifically depends on comparing what was planned with what was eventually reported, however, the protocol may be central.
The appropriate response is therefore proportional. Do not reject an entire study merely because an appendix is unavailable, but do not claim that a methodological detail was prespecified if the evidence required to establish that point is inaccessible.
04 · A Practical Example
Tracing a Protocol That Is Mentioned but Not Attached
Hypothetical Example
The Methods say “according to the registered protocol,” but you cannot find it
Suppose you are evaluating a study and want to determine whether its primary outcome was prespecified. The paper repeatedly refers to a protocol, but the PDF contains no appendix and the protocol is not immediately visible on the journal page.
1. Identify the reference Search the Methods, references, registration statement, and supplementary-information section for a protocol citation, DOI, or registration number.
2. Inspect the publisher record Open the current article page and check all supplementary and associated-material links rather than relying solely on the PDF.
3. Follow the identifier If a registry number or protocol citation is supplied, use that identifier to locate the corresponding record directly.
4. Verify the match Compare investigators, study characteristics, dates, registration identifiers, and other details before concluding that the document belongs to the published study.
5. Escalate the search if necessary Check legitimate repositories and contact the corresponding author or publisher if the document still cannot be located.
6. Limit the conclusion If the protocol remains unavailable, report that you could not verify prespecification from the protocol rather than assuming that the published outcome was or was not prespecified.
The final step is crucial. “I could not locate the protocol” and “the investigators did not prespecify the outcome” are not equivalent claims. Absence from your search is not evidence of absence from the study's history.
07 · A Quick Checklist
Before Concluding That an Appendix or Protocol Is Unavailable
Before treating the document as unavailable, check:
Record the exact appendix, protocol, or supporting document named in the paper.
Search the Methods, references, end matter, registration statement, and data-availability statement for identifiers.
Inspect the current publisher landing page and all supplementary-material sections.
Resolve any DOI, registration number, accession number, or repository identifier directly.
Search for a separately published protocol when the paper appears to cite one.
Check legitimate institutional, disciplinary, or study repositories where appropriate.
Verify that any candidate protocol or appendix actually corresponds to the study.
Contact the corresponding author or publisher when the document should exist but cannot be located.
Document the search if the missing material affects your analysis or appraisal.
