Most researchers pick a citation style the way they pick a template: whatever the target journal or department requires, formatted with a reference manager, and never thought about again. That’s the right instinct — style choice is rarely a free decision, and this page won’t argue otherwise. What it does cover is the part that actually needs judgment: which style applies by default in which field, what the major styles genuinely differ on structurally (not just punctuation trivia), how to actually format the same source across all of them, and where to look when a journal’s instructions are silent or ambiguous.
This page is deliberately scoped to style selection, structural differences, and the mechanical formatting questions researchers search for individually — running heads, footnote punctuation, how to cite a source with no listed author. For DOI-specific formatting, CASRAI covers that in more depth in How to Find, Cite, and Use a DOI, and How to Cite a Dataset covers the distinct rules for data and software outputs.
Two underlying systems, not seven unrelated ones
Nearly every citation style in wide use is a variant of one of two underlying systems, and understanding that split makes the whole landscape learnable rather than something to memorize style by style:
- Author-date (parenthetical) systems — the reader sees a name and year (or name and page) inline, then looks it up alphabetically in a reference list. APA, MLA, Harvard, and Chicago’s author-date option all work this way.
- Numeric systems — each source gets a number the first time it’s cited, and that number is reused every time; the reference list is ordered by citation number, not alphabetically. Vancouver/ICMJE, AMA, and IEEE all work this way, and ACS allows it as one of its options.
Chicago is the outlier that spans both: its notes-bibliography system is a third approach again (see the footnote section below), and its author-date option is functionally the same family as APA and Harvard.
The major styles at a glance
| Style | Current edition | Governing body | Typical fields | System |
|---|---|---|---|---|
| APA | 7th (2020) | American Psychological Association | Psychology, education, social and behavioral sciences, nursing, business | Author-date |
| MLA | 9th | Modern Language Association | Literature, language, and the humanities | Author-page (author-date family) |
| Chicago | 18th (2024) | University of Chicago Press | History and humanities (notes-bibliography); sciences and social sciences (author-date) | Both — notes-bibliography or author-date |
| IEEE | Current IEEE Reference Guide | Institute of Electrical and Electronics Engineers | Engineering, computer science, electronics | Numeric, order of citation |
| ACS | 3rd (ACS Style Guide) | American Chemical Society | Chemistry and related physical sciences | Author’s choice: superscript, parenthetical number, or author-date |
| Vancouver / ICMJE (NLM) | Per NLM’s Citing Medicine, 2nd ed. | International Committee of Medical Journal Editors, following National Library of Medicine standards | Biomedical and clinical medicine | Numeric, order of first mention |
| Harvard | No single fixed edition — institution- or publisher-specific | No single governing body | Common across UK, Australian, and some European institutions; general sciences and social sciences | Author-date |
A field default is not a rule, and assuming it settles the question is the single most common style-selection mistake: a target journal’s own instructions for authors always override the field convention. Plenty of psychology journals don’t use APA; plenty of engineering venues don’t use IEEE. Check the specific journal, publisher, or department’s style guide before assuming the disciplinary default applies.
Detailed formatting comparison
Below the surface-level punctuation differences, the styles differ on a small number of structural choices that change how a reference list actually looks:
| Style | In-text form | Reference list order | Author name format | Title capitalization | Where the year sits |
|---|---|---|---|---|---|
| APA | (Author, Year) | Alphabetical by surname | Surname, Initials | Sentence case for article/book titles; title case for journal names | Immediately after author, in parentheses |
| MLA | (Author Page) | Alphabetical by surname | Surname, First Name (full name, first entry only) | Title case for all titles | Near the end of the Works Cited entry |
| Chicago (author-date) | (Author Year, Page) | Alphabetical by surname | Surname, First Name | Title case | Immediately after author |
| Chicago (notes-bibliography) | Superscript number → footnote/endnote | Alphabetical (bibliography) or citation order (notes) | First Name Surname (note); Surname, First Name (bibliography) | Title case | Mid-entry, after publisher |
| IEEE | [1] | Order of first citation | Initials Surname | Sentence case for article titles | Near the end, after the venue |
| ACS | Superscript1, (1), or (Author, Year) | Citation order (numeric) or alphabetical (author-date) | Surname, Initials | Sentence case for article titles; abbreviated journal names | After the abbreviated journal name |
| Vancouver/ICMJE | [1] or (1), superscript in many journals | Order of first citation | Surname Initials (no comma, no periods in initials) | Sentence case | After the abbreviated journal name |
| Harvard (typical) | (Author, Year) | Alphabetical by surname | Surname, Initials | Sentence case (varies by institution) | Immediately after author |
Harvard’s row is deliberately marked “typical” rather than fixed — see below.
Harvard is a family of styles, not one fixed standard
This is worth stating plainly because it’s a genuine, frequent source of confusion: unlike APA, MLA, or Chicago, “Harvard referencing” is not maintained by a single body and has no single canonical edition. It refers to a broad family of author-date referencing conventions, widely taught and used across UK, Australian, and some European universities, that share the same basic (Author, Year) in-text pattern and alphabetical reference-list structure as APA, but differ in the small mechanical details — punctuation, whether “and” or “&” joins multiple authors, how page ranges are formatted — from one institution’s or publisher’s version to the next. Some universities publish their own named variant (Cite Them Right is a common commercially published version used as an institutional standard in the UK). Because there is no single authoritative Harvard style guide, the only reliable source for exact formatting is the specific institution’s or journal’s own style document, not a generic “Harvard style” description found online.
Style by style
APA (American Psychological Association), 7th edition
Originated in 1929 among a group of psychologists, anthropologists, and business managers who wanted standardized reporting conventions for scientific writing. APA’s own guidance describes its use across “psychology, nursing, business, communications, engineering, and related fields,” plus the broader social and behavioral sciences, health care, and natural sciences. Author-date, parenthetical in-text citation; alphabetical reference list.
MLA (Modern Language Association), 9th edition
The default for literature, language studies, cultural studies, and most humanities coursework and journals. Author-page parenthetical in-text citation — the page number matters because MLA scholarship frequently quotes and discusses specific passages; alphabetical Works Cited list.
Chicago (The Chicago Manual of Style), 18th edition
Unusual among the major styles in offering two genuinely different systems rather than one. The notes-bibliography system — footnotes or endnotes plus a bibliography — is Chicago’s oldest and most flexible option, predominantly used in history, literature, and the arts, where the flexibility to add discursive notes alongside citations is valued. The author-date system — parenthetical citations plus a reference list — is preferred in the sciences and social sciences. Which system applies is a real decision a Chicago-style manuscript has to make explicitly; “Chicago style” alone doesn’t specify one.
IEEE
The standard for engineering, computer science, and electronics publishing. Numbered citations in square brackets, ordered by first appearance in the text, with the reference list itself ordered by citation number rather than alphabetically — this is IEEE’s most distinctive practical difference from citation-manager defaults built primarily around APA/MLA-style alphabetical lists, and worth checking explicitly if switching a reference manager’s output style. IEEE reference lists also commonly retain a bare doi: prefix rather than the full https://doi.org/ resolver-link form most other current style guidance has converged on — a genuine, current exception rather than an error if encountered in a published IEEE paper.
ACS (American Chemical Society)
Governed by the ACS Style Guide, now in its 3rd edition, and used across ACS’s own journal portfolio (such as the Journal of the American Chemical Society and Analytical Chemistry) and widely elsewhere in chemistry publishing. ACS is unusual among the major styles in that it doesn’t mandate one in-text approach: authors may cite sources with a superscript number, a number in parentheses, or an author-date parenthetical — all three are acceptable ACS formats, but a manuscript must pick one and use it consistently throughout. Whichever numeric option is chosen, the reference list is ordered by citation order; author-date citations are ordered alphabetically instead. ACS reference-list entries abbreviate journal titles according to the Chemical Abstracts Service (CAS) source index abbreviations, which is a common formatting trap for authors used to spelling journal names out in full.
Vancouver / ICMJE (NLM), per Citing Medicine
ICMJE’s own current recommendations state that references “should be numbered consecutively in the order in which they are first mentioned in the text,” and direct authors to the National Library of Medicine’s Sample References and Citing Medicine (2nd edition) rather than prescribing every formatting detail itself — Vancouver/NLM is a shared, evolving standard rather than a single fixed rulebook, which is part of why exact conventions (DOI placement, in particular) vary somewhat by journal even within the same style family. Used across most biomedical and clinical journals, including many that don’t otherwise identify as following “ICMJE recommendations” by name. AMA (the AMA Manual of Style, 11th edition, the house style of JAMA and the JAMA Network journals) is a separate, more prescriptive numbered style that is often confused with Vancouver because both trace back to the same medical-editing tradition — check which one a specific journal names, since a citation formatted correctly in one won’t always pass in the other.
Harvard
See the dedicated section above — Harvard is a widely used author-date family rather than a single fixed standard, and the only reliable source for a specific journal or institution’s exact Harvard conventions is that journal or institution’s own published style document.
Worked example: the same four sources in every style
Illustrative examples only — these are generic, invented placeholder sources built to demonstrate formatting, not real publications. Substitute your own source’s actual metadata.
A journal article
- APA: Alvarez, T. M., & Berg, K. (2023). Measuring reproducibility in laboratory protocols. Journal of Research Methods, 41(2), 118–134. https://doi.org/10.xxxx/xxxxx
- MLA: Alvarez, Tomás M., and Karin Berg. “Measuring Reproducibility in Laboratory Protocols.” Journal of Research Methods, vol. 41, no. 2, 2023, pp. 118–34.
- Chicago (author-date): Alvarez, Tomás M., and Karin Berg. 2023. “Measuring Reproducibility in Laboratory Protocols.” Journal of Research Methods 41 (2): 118–34.
- Chicago (notes): 1. Tomás M. Alvarez and Karin Berg, “Measuring Reproducibility in Laboratory Protocols,” Journal of Research Methods 41, no. 2 (2023): 118–34.
- IEEE: [1] T. M. Alvarez and K. Berg, “Measuring reproducibility in laboratory protocols,” J. Res. Methods, vol. 41, no. 2, pp. 118–134, 2023.
- ACS (numeric): Alvarez, T. M.; Berg, K. Measuring Reproducibility in Laboratory Protocols. J. Res. Methods 2023, 41, 118–134.
- Vancouver/ICMJE: 1. Alvarez TM, Berg K. Measuring reproducibility in laboratory protocols. J Res Methods. 2023;41(2):118–34.
- Harvard (typical): Alvarez, T.M. and Berg, K. (2023) ‘Measuring reproducibility in laboratory protocols’, Journal of Research Methods, 41(2), pp. 118–134.
A book
- APA: Okafor, R. (2021). Principles of experimental design. Ashgrove Press.
- MLA: Okafor, Rebecca. Principles of Experimental Design. Ashgrove Press, 2021.
- Chicago (notes): 2. Rebecca Okafor, Principles of Experimental Design (Chicago: Ashgrove Press, 2021), 44.
- Vancouver/ICMJE: 2. Okafor R. Principles of experimental design. Chicago: Ashgrove Press; 2021.
- Harvard (typical): Okafor, R. (2021) Principles of experimental design. Chicago: Ashgrove Press.
A book chapter
- APA: Nakamura, S. (2020). Sampling error in field studies. In L. Petrov (Ed.), Handbook of applied statistics (pp. 210–229). Camden Academic.
- MLA: Nakamura, Sana. “Sampling Error in Field Studies.” Handbook of Applied Statistics, edited by Lena Petrov, Camden Academic, 2020, pp. 210–29.
- Chicago (notes): 3. Sana Nakamura, “Sampling Error in Field Studies,” in Handbook of Applied Statistics, ed. Lena Petrov (Camden: Camden Academic, 2020), 214.
A website (no fixed author or publication date)
This is one of the most frequently searched formatting questions, because it’s the case general style guidance covers least clearly. The underlying principle is the same across every style: use whatever is actually available, in order of preference, and substitute a standard placeholder for what’s missing.
- No individual author: use the organization or site name as the author in its place (e.g., “World Health Organization” or “National Institutes of Health” rather than leaving the author field blank).
- No date: APA and Chicago use “n.d.” (no date) in place of the year; MLA generally omits the date field entirely rather than guessing.
- Example (APA): National Institutes of Health. (n.d.). Research data sharing policy overview. https://www.example.gov/data-sharing
- Example (MLA): “Research Data Sharing Policy Overview.” National Institutes of Health, www.example.gov/data-sharing. Accessed 3 Aug. 2026.
Before citing a source with no clear author or date at all, it’s worth checking whether it’s actually a reliable source to cite — see CASRAI’s CRAAP test guide for evaluating source credibility.
Footnotes, endnotes, and parenthetical systems
These three mechanisms are often used loosely as if interchangeable, but they do different jobs:
- Parenthetical (in-text) citations — a short author-date or author-page marker sits directly in the sentence, e.g. (Alvarez & Berg, 2023). Used by APA, MLA, Chicago author-date, and Harvard.
- Footnotes — a superscript number in the text points to a note printed at the bottom of the same page. Used by Chicago’s notes-bibliography system and common in law and some humanities styles.
- Endnotes — functionally identical to footnotes, but all notes are collected at the end of the chapter or document rather than at the foot of each page. Chicago permits either; which one a manuscript uses is usually a publisher house-style choice rather than an author’s.
Footnote punctuation conventions that trip people up most often: the superscript note number is placed after the punctuation mark it follows (after a period or comma, not before it) — “…reported in the original protocol.¹” is correct; “…reported in the original protocol¹.” is not. Within the footnote or endnote itself, Chicago’s notes format uses commas rather than the periods a bibliography entry for the same source would use, and the author’s name is given in normal First-Name-Surname order in a note, reversed to Surname, First-Name only in the bibliography — the same source is formatted two different ways depending on which list it appears in, which is a common source of inconsistency in first drafts.
APA running head and title page requirements
The APA running head is one of the most frequently searched individual formatting questions, and the 7th edition genuinely changed the rule in a way that trips up anyone working from older guidance: student papers no longer require a running head at all under APA 7th edition, unless a specific instructor or institution requires one. Professional papers (manuscripts intended for publication) still require a running head — a shortened version of the title, in capital letters, in the page header, left-aligned, with the page number right-aligned, and no longer required to include the literal label “Running head:” that older (6th edition) guidance used.
Title-page requirements also differ by paper type under APA 7th edition: a student title page needs the paper title, author name(s), institutional affiliation, course number and name, instructor, and due date; a professional title page needs the title, author name(s) and affiliation(s), an author note (funding, disclosures, and contact information), and the running head described above. CASRAI’s guide to writing a research paper in APA style covers full manuscript structure beyond just the title page.
What researchers actually need to know day to day
- The target journal’s guidelines override the general style. A journal’s own instructions for authors are authoritative even when they deviate from what’s typical for the field or from the general style manual’s own default recommendation.
- Reference managers do the formatting; clean metadata is the real skill. Zotero, Mendeley, and EndNote apply Citation Style Language (CSL) definitions — a public, machine-readable schema covering the major styles above plus thousands of individual journal house styles — to structured bibliographic metadata (author, title, container, date, identifier). Almost no one hand-formats reference lists across multiple styles anymore. The output is only ever as accurate as the metadata feeding it: a garbled or incomplete source record produces a garbled citation regardless of which style is selected, so verifying metadata before generating a bibliography matters more than memorizing punctuation rules.
- DOIs are increasingly formatted as full resolver URLs. Current APA, MLA, and Chicago guidance favors the full
https://doi.org/10.xxxx/xxxxxform over a bare DOI string; IEEE and some journal house styles are the notable holdouts still using a baredoi:prefix, so check the specific style guide rather than assuming one convention is universal.
A mangled citation is a reporting error, not a formatting one
It’s worth separating two very different kinds of citation problem. Getting the punctuation, capitalization, or list order wrong for a given style is a formatting issue — cosmetic, and usually caught at copyediting. Citing a source that doesn’t actually say what the citing sentence claims it says, misattributing a finding to the wrong study, or citing a source secondhand without having read it, is a different category of problem entirely: a misrepresentation of the evidence, which is a research-integrity concern regardless of whether the reference-list entry itself is formatted correctly. CASRAI’s guide to quoting and citing sources correctly covers this distinction and how to avoid misattribution when paraphrasing or quoting; see also primary vs. secondary sources for the related problem of citing a claim through an intermediate source rather than the original.
How to actually decide which style to use
- Check the target journal or publisher’s instructions for authors first. This overrides every field convention listed above.
- Check your institution’s or department’s requirement for coursework, theses, and dissertations where there’s no external journal to defer to — this is where a specific named Harvard variant, in particular, matters.
- If neither applies, use the field default from the table above as a reasonable starting point, but confirm with a supervisor or editor before treating it as settled.
- For biomedical work specifically, confirm Vancouver vs. AMA rather than assuming — the two are close enough to cause real formatting errors if conflated.
Frequently asked questions
What’s the actual difference between APA and MLA?
Both use parenthetical in-text citations and an alphabetical reference list, so they look superficially similar. APA leads with the publication year and cites by author-date; MLA leads with the source container and cites by author-page, since page numbers matter more when close reading and quoting specific passages, as in literary analysis. Check the target journal, publisher, or institution’s instructions rather than assuming field convention settles it.
Does ACS use numbers or author-date citations?
Either — ACS is unusual among the major styles in explicitly allowing three in-text options (superscript number, parenthetical number, or author-date), leaving the choice to the author or journal, provided it’s used consistently throughout the manuscript. Check the specific journal’s author guidelines, since many ACS journals have their own preference despite the style guide itself allowing all three.
Is Harvard style the same everywhere?
No. Unlike APA, MLA, or Chicago, there is no single body that maintains “Harvard style,” so exact conventions (punctuation, how multiple authors are joined, date placement) vary by institution and publisher. Always check the specific style guide named by the assigning institution or journal rather than a generic online description of “Harvard referencing.”
What’s the difference between a footnote and an endnote?
Both point from a superscript number in the text to a fuller citation or comment; a footnote appears at the bottom of the same page, while an endnote is collected with all other notes at the end of the chapter or document. Chicago’s notes-bibliography system permits either, and which one is used is typically a publisher house-style decision.
Do I need a running head in APA format?
Under APA 7th edition, student papers do not require a running head unless an instructor specifically asks for one; professional papers intended for publication still require one, formatted as a shortened title in capital letters in the page header, without the literal “Running head:” label used in the older 6th edition.
How do I cite a website with no listed author or date?
Use the organization or site name in place of an individual author, and use “n.d.” (no date) in place of the year in APA and Chicago, or omit the date field in MLA. Verify the source is reliable enough to cite in the first place before formatting it — see CASRAI’s CRAAP test guide.
Is Vancouver style the same as AMA style?
No, though they’re frequently confused. Both are numbered systems that trace back to the same ICMJE/medical-editing tradition, but Vancouver (formally, NLM’s Citing Medicine) and the AMA Manual of Style (11th edition) are separate, independently maintained style guides with real formatting differences. Confirm which one a specific journal names in its instructions for authors.
Does it matter which citation style I use if I’m using a reference manager like Zotero or EndNote?
The manual formatting work is automated, but the underlying choice still matters — you still have to select the correct style file, and the output is only as accurate as the metadata you (or the source’s DOI/database record) provided. Reference managers apply Citation Style Language (CSL) definitions to structured metadata; garbled or incomplete source metadata produces a garbled citation regardless of which style is selected.
How do I cite a DOI correctly in each of these styles?
See CASRAI’s How to Find, Cite, and Use a DOI guide, which has full worked reference-list examples in APA, MLA, Chicago, Vancouver, and IEEE format, plus a quick-reference table of DOI placement and formatting conventions by style.
