Chicago style is the referencing system of The Chicago Manual of Style (17th edition), offering two variants — notes-bibliography, which uses footnotes or endnotes, and author–date, which uses parenthetical citations. Vancouver style, codified in the ICMJE Recommendations, is a numeric system in which sources are numbered in the order they first appear and listed in that numeric order. Chicago dominates history, the arts and publishing; Vancouver is standard across medicine and the biomedical sciences.
These two styles bracket the range of approaches a researcher will meet. Chicago is the most flexible of the major systems; Vancouver is the most compact. Seeing them side by side clarifies why a discipline chooses one referencing logic over another — a theme we develop in our broader comparison of APA, MLA, Chicago and Vancouver.
Chicago notes-bibliography
In the notes-bibliography variant, a superscript number in the text points to a footnote (or endnote) carrying the full source details, and a parallel bibliography at the end lists all sources alphabetically. The note and bibliography entry differ slightly in format. A first footnote gives full detail:
1. Jane Smith, Foundations of Research Integrity (London: Academic Press, 2019), 142.
The matching bibliography entry inverts the author’s name and uses full stops rather than commas and brackets:
Smith, Jane. Foundations of Research Integrity. London: Academic Press, 2019.
Subsequent references to the same work are shortened to author, short title and page — Smith, Foundations, 88 — which keeps the notes readable. This variant suits the humanities because footnotes can carry commentary as well as citations, letting a historian discuss a source without breaking the main argument.
Chicago author–date
The author–date variant works much like APA: a parenthetical (Smith 2019, 142) in the text points to an alphabetical reference list. It is favoured in the sciences and social sciences when Chicago is the house style. The reference-list entry resembles the bibliography entry but moves the year forward: Smith, Jane. 2019. Foundations of Research Integrity. London: Academic Press. Choosing between Chicago’s two variants is normally dictated by your discipline or publisher, not personal preference.
Vancouver and the ICMJE Recommendations
Vancouver style assigns each source a number the first time it is cited, in brackets or as a superscript — [1] or 1 — and reuses that number on every later citation of the same source. The reference list is then ordered numerically, by order of first appearance, not alphabetically. The ICMJE Recommendations specify the format, including abbreviated journal titles and a distinctive author style with no full stops after initials:
1. Smith J, Jones R. Open-access uptake in clinical trials. J Res Stand. 2021;14(3):220–38.
Vancouver’s numeric compactness suits biomedical papers, which often carry dozens of references and value an unobtrusive in-text marker. Because the numbering follows appearance order, inserting a new citation mid-draft renumbers everything after it — which is why reference-manager software is near-universal in the field.
Comparing the three at a glance
| Feature | Chicago notes-bib | Chicago author–date | Vancouver |
|---|---|---|---|
| In-text marker | Superscript note number | (Author Year, page) | Bracketed number [1] |
| List order | Alphabetical bibliography | Alphabetical reference list | Numeric, by first appearance |
| Typical disciplines | History, arts, publishing | Sciences, social sciences | Medicine, biomedical sciences |
| Governing standard | Chicago Manual 17th ed. | Chicago Manual 17th ed. | ICMJE Recommendations |
Why the differences matter
The choice of system is not arbitrary decoration. A footnote system lets a humanities scholar annotate and qualify a source in place; a numeric system lets a clinician cram a dense evidence base into a tight word limit; an author–date system keeps the year visible where recency is part of the argument. Each encodes a different relationship between the writer’s text and the scholarly record. Underneath all of them, the obligations to assign credit accurately and to record authorship honestly are the same — only the surface format changes.
If you are moving between disciplines and meeting an unfamiliar style, CASRAI’s guidance for authors can help you map an output onto whichever convention a new editor requires.
Frequently asked questions
Is Chicago author–date the same as APA?
They are close cousins but not identical. Both use author–date in-text citations and an alphabetical list, but punctuation, capitalisation and the treatment of titles differ. Chicago title-cases titles and has its own publisher conventions; APA uses sentence case for article titles. Always follow the specific manual your editor names.
How does Vancouver handle a source cited several times?
It keeps the original number. A source numbered [3] on first appearance is cited as [3] every subsequent time, and appears once in the reference list at position 3. The number is fixed to the source, not to the location of the citation.
When should I use notes-bibliography rather than author–date Chicago?
Use notes-bibliography in the humanities and wherever footnotes are expected to carry discussion as well as citations. Use author–date in the sciences and social sciences, or wherever a journal’s house style specifies it. The decision is set by the publisher, not the writer.
Do these styles require DOIs?
Both increasingly expect a DOI where one exists, and the ICMJE Recommendations encourage including it for journal articles. A DOI anchors the entry to a persistent address, which matters as much in numeric and notes systems as in author–date ones.
