Scientific papers primarily use two types of citation systems: numerical citations, where references appear as superscript or bracketed numbers in the text, and author-date citations, where the author’s last name and publication year appear in parentheses. Which system a paper uses depends on the scientific discipline and the specific journal’s requirements. Several major style guides govern these formats, each with its own rules for how references look both in the text and in the reference list at the end.
Numerical vs. Author-Date Systems
The fundamental split in scientific citations comes down to how sources are flagged within the text itself. In a numerical system, each source gets a number (1, 2, 3) that appears either as a superscript or in brackets. The reader then looks up that number in a numbered reference list at the end of the paper. In an author-date system, the citation appears as something like (Smith 2021) right in the sentence, and the reference list at the end is organized alphabetically by author name.
Physics, engineering, and medical journals tend to favor numerical systems because they keep the text clean and uncluttered, especially when a single sentence might cite five or six sources. Ecology, psychology, and the social sciences lean toward author-date because seeing a researcher’s name and publication year directly in the text helps readers immediately gauge who did the work and how recent it is. Neither system is inherently better. Journals choose based on the reading conventions of their field.
Vancouver Style
Vancouver is the dominant citation style in biomedical and medical research. It uses a straightforward numerical system: sources are numbered consecutively in the order they first appear in the text using Arabic numerals. If you cite a source on page 2 and then cite it again on page 8, it keeps the same number both times. The reference list at the end is arranged in that same numerical order, not alphabetically. Each number corresponds to exactly one source.
Vancouver formatting was developed alongside the guidelines from the International Committee of Medical Journal Editors (ICMJE), and it’s built on the citation standard maintained by the National Library of Medicine. A typical Vancouver reference looks like this: the authors’ surnames and initials, the article title, the abbreviated journal name, the publication date, volume, issue, and page numbers, followed by a PubMed identification number when available. Most major medical journals, including those indexed in PubMed, use this format or a close variant of it.
Council of Science Editors (CSE) Style
CSE style is widely used in the biological sciences and offers three distinct systems within a single style guide, which makes it unusually flexible.
- Citation-sequence: Works like Vancouver. Sources are numbered in the order they first appear in the text, and the reference list follows that same order. If the first source you cite is by Smith, Smith is reference number 1.
- Citation-name: The reference list is arranged alphabetically by the first author’s last name, and each entry gets a number based on its alphabetical position. Those numbers are then used in the text. So if Zielinski is number 56 in the alphabetical list, every in-text reference to that source is 56, regardless of where it appears in the paper.
- Name-year: An author-date system. In-text references include the author’s surname and publication year in parentheses. The reference list is unnumbered and arranged alphabetically, with multiple works by the same author listed in chronological order.
Which of the three systems a paper uses is determined by the journal. Authors typically check the journal’s submission guidelines before formatting their references.
APA Style in the Sciences
APA (American Psychological Association) style is an author-date system most commonly associated with psychology and the social sciences, but it’s also used in nursing, public health, education research, and some interdisciplinary science journals. In-text citations list the author’s last name and year of publication, such as (Johnson, 2023), and the reference list at the end is organized alphabetically.
The current version, the 7th edition, made several practical changes. Reference entries now include up to 20 authors. For sources with more than 20, you list the first 19, add an ellipsis, and then the final author’s name. Digital object identifiers (DOIs) are now formatted as clickable hyperlinks rather than preceded by the label “DOI:” as in earlier editions. These changes reflect the shift toward digital-first publishing, where readers access papers online and need working links to track down sources.
NLM Style and PubMed Citations
The National Library of Medicine maintains its own citation format, described in its publication “Citing Medicine.” This format serves as the foundation for how millions of journal articles are indexed and displayed in PubMed, the largest database of biomedical literature. A typical NLM-formatted citation includes the authors, article title, abbreviated journal name, full publication date, volume, issue, page range, and PubMed ID (PMID), a unique number that lets anyone find the article instantly in the database.
NLM style follows the ANSI/NISO Z39.29 bibliographic standard, which is essentially a national rulebook for how references should be structured. Because the ICMJE recommendations for medical journals are built on NLM formatting, researchers publishing in clinical medicine encounter this style constantly, even if their journal officially calls it “Vancouver style.” The two are closely related, with NLM providing the more detailed technical specification.
How Journals Enforce Their Chosen Style
Every scientific journal publishes an “Instructions for Authors” or “Guide for Authors” page that specifies exactly which citation style to use, down to details like punctuation, abbreviation of journal names, and whether to include issue numbers. Submitting a paper in the wrong format can delay the review process or result in a desk rejection before peer review even begins.
Most researchers use reference management software to handle formatting automatically. Tools like Zotero, Mendeley, and EndNote store citation data in standardized file formats (such as BibTeX .bib files or RIS files) that can export references in thousands of journal-specific styles at the click of a button. For researchers using LaTeX to write their papers, pairing a .bib file with the correct style template produces a fully formatted reference list with minimal manual editing. This automation is essential because a single researcher might submit to journals that use completely different citation systems over the course of a year.
Citing AI Tools in Scientific Papers
As researchers increasingly use AI tools like ChatGPT in their workflows, citation guidelines are catching up. APA style now includes guidance for citing generative AI, treating it as a source that should be referenced in-text and included in the reference list. However, the rules vary significantly depending on the publisher, the journal, and even individual course instructors in academic settings. Some journals require disclosure of any AI use in a dedicated section of the paper, while others prohibit AI-generated text entirely. If you’re writing a scientific paper and used AI tools during any part of the process, check your target journal’s specific policy before submitting.