Do I need to include a DOI in every bibliography entry?

Only when a DOI exists. For journal articles and datasets, a DOI is required in most modern citation styles (APA 7, IEEE, AMA, Vancouver) whenever one has been assigned. Books use an ISBN instead. Older articles published before roughly 2000 often have no DOI — in that case, use a stable URL or omit the link entirely.

What is the difference between ISBN-10 and ISBN-13?

ISBN-13 (13 digits, e.g. 978-3-518-29483-5) is the current standard; ISBN-10 (10 digits) was used for books published before 2007. Both are valid identifiers, but modern bibliographies prefer the 13-digit form. Most reference managers like Zotero accept either format.

Does Zotero fill in DOI, ISBN, and ISSN automatically?

Usually yes. Zotero pulls DOI and ISBN from metadata when you add a source, and you can trigger a manual lookup via right-click → 'Retrieve metadata for PDF' or 'Add item by identifier'. Always double-check the populated fields — in particular whether the DOI is in the correct format for your citation style (plain identifier vs. full https://doi.org/... URL).

DOI, ISBN, ISSN — what actually belongs in a citation

Anyone writing an academic thesis will sooner or later encounter three abbreviations: DOI, ISBN, ISSN. Some citation styles require all three, others only one, and Zotero fills them in automatically most of the time — as long as you know when you need to add one yourself. This post explains each abbreviation briefly, shows you where to find it, and tells you when it belongs in your bibliography.

DOI — Digital Object Identifier

A DOI is the digital address of a scholarly publication. It looks like this: 10.1038/s41586-022-04958-w. DOIs are assigned centrally by Crossref (for journals and books) and DataCite (for datasets) — and they remain valid even if the publisher moves the URL. That’s the point: a DOI is not a link, but any DOI resolves via https://doi.org/<doi>.

When do you need a DOI in your bibliography?

For journal articles: in almost all modern citation styles (APA 7, Chicago Author-Date, AMA, JAMA, IEEE) required if available.
For conference proceedings and book chapters: increasingly required if a DOI exists.
For datasets, preprints (arXiv, bioRxiv), software: required.

Where do you find the DOI?

On the publisher’s article page — usually top right or in the citation info.
In the PDF itself on the first page, printed small below the author names or in the footer area.
Via crossref.org using the title or authors.
Directly in Zotero via right-click → “Retrieve metadata for DOI/ISBN” — if the DOI appears in the PDF, Zotero pulls it automatically.

Pitfall: There are two DOI formats: just the identifier (10.1038/...) and as a URL (https://doi.org/10.1038/...). APA 7 requires the URL form with https://doi.org/. Check your style guide — Zotero renders the correct form automatically when the field is properly populated.

ISBN — International Standard Book Number

An ISBN identifies a specific book in a specific edition and language. The modern format is 13 digits, e.g. 978-3-518-29483-5. Older books still have 10-digit ISBNs — both are valid, but newer bibliographies prefer the 13-digit version.

When do you need an ISBN in your bibliography?

In most humanities styles (Chicago Notes-Bibliography, MLA): optional, often omitted.
In social sciences / psychology (APA 7): not required for standard monographs.
In legal writing (German tradition): often recommended, because edition identification matters.
For e-books with their own ISBN: if you’re reading the e-book, cite the e-book ISBN (not the print ISBN).

Where do you find the ISBN?

On the verso of the title page in any book.
On the publisher’s website and in any library catalogue.
In Zotero via right-click → “Retrieve metadata for DOI/ISBN” — works with ISBN too.

Pitfall: Different editions have different ISBNs. If you’re citing the 2nd edition, the 2nd edition’s ISBN belongs in your bibliography — not the 1st edition’s.

ISSN — International Standard Serial Number

An ISSN identifies a journal (serial), not an individual article. Format: XXXX-XXXX, e.g. 0028-0836 (Nature). There are print ISSNs and electronic ISSNs (eISSN) — some journals have both.

When do you need an ISSN in your bibliography?

In most social science and humanities styles (APA, Chicago, MLA): no.
In medicine and natural sciences (Vancouver, AMA): sometimes.
In legal writing (German tradition): no.

Rule of thumb: if you’re unsure whether the ISSN belongs, check your style guide. In over 80% of student theses, the ISSN is superfluous.

Where do you find the ISSN?

On the journal’s title page, often in the masthead.
On the journal’s publisher page (not the article page).
Via the ISSN database portal.issn.org.

Which identifier takes priority?

If a journal article has both a DOI and an ISSN (which is usually the case), the DOI serves as the unique identifier for the article and takes precedence. For a book with both an ISBN and a DOI, most styles also favour the DOI. The rule is simple: the more specific the identifier, the more important. DOI > ISBN > ISSN.

What happens when a DOI is missing?

Some older articles have no DOI — typically everything published before around 2000. Two options:

Provide a stable link. If the article is stably accessible via JSTOR, Project MUSE, or a similar platform, give the “stable URL” (not the session URL, which becomes invalid once the session ends).
Omit the link entirely. APA 7 permits this. You then list only the standard fields (author, year, title, journal, volume, issue, pages).

What you should not do: cite the publisher’s direct URL for an article that has a DOI. That URL can change; the DOI cannot.

How Acurio uses these fields

When verifying citations with Acurio, the DOI is the strongest signal you can provide: using the DOI, Acurio can identify whether you’ve cited the correct edition of a source and cross-check the source PDF against the DOI metadata. A clean Zotero library with populated DOI and ISBN fields makes the verification noticeably more precise.

In brief: DOI whenever available — for articles and datasets. ISBN only for books, and only where your style requires it. ISSN almost never in student theses. And Zotero saves you 90% of the manual entry if you use the DOI/ISBN lookup function.