An apostrophe does two jobs, and never makes anything plural
It shows possession, or it marks a missing letter. That is the whole list. Nearly every apostrophe error in academic writing comes from using one for a third job it does not have, which is forming a plural.
Job one: possession
Add apostrophe s to a singular noun, whether or not it already ends in s.
Singular possessive
the student's results, the university's policy, the researcher's notes
Jones's data, the class's average (APA 7 adds apostrophe s even after s)
For a plural that already ends in s, put the apostrophe after that s and add nothing.
Plural possessive
the students' results, three universities' policies, the participants' responses
For irregular plurals that do not end in s, treat them like singulars.
Irregular plural possessive
the children's scores, the women's group, the people's response
The reliable method is to form the plural first, then decide where the apostrophe goes. Write “students”, see that it ends in s, put the apostrophe after it. Write “children”, see that it does not, add apostrophe s.
Job two: a missing letter
An apostrophe marks where letters have been removed. Do not is contracted to don’t, it is to it’s, cannot to can’t.
In formal academic writing you generally should not use contractions at all. Writing them out in full is more formal, and it also eliminates most apostrophe decisions from the document before they arise.
The error that accounts for most of the rest
An apostrophe does not make anything plural. Not nouns, not numbers, not abbreviations, not decades, not names.
Wrong
The participant's were recruited from three site's during the 1990's, and several RCT's were reviewed.
Right
The participants were recruited from three sites during the 1990s, and several RCTs were reviewed.
This includes family names on cards and signs, which is where most people first learn the habit. The Okafors, not the Okafor’s.
Its and it’s, which is worth memorizing once
This single pair produces more apostrophe errors than everything else combined, because it runs against the rule people have just learned.
It’s means it is or it has. The apostrophe marks the missing letter.
Its is possessive. It takes no apostrophe, because possessive pronouns never do.
Both, in one sentence
It's clear that the instrument has limitations, and its reliability has not been established in this population.
The test is expansion. If “it is” fits, use the apostrophe. If it does not, leave it out.
The same logic applies across the possessive pronouns: yours, hers, theirs, ours and whose all take no apostrophe. Who’s, on the other hand, means who is.
Joint and separate possession
When two people own something together, only the second name takes the apostrophe. When they own separate things, both do.
Joint: one study by two authors
Okafor and Raine's study
Separate: two studies
Okafor's and Raine's studies
A search that finds most errors in a minute
Search your document for the apostrophe character and look at each one. For each, ask which of the two jobs it is doing.
If it is possession, check whether the noun is singular or plural and whether the apostrophe is on the correct side of the s.
If it is a contraction, consider writing the words out in full instead, since this is academic writing.
If it is neither, it is almost certainly an apostrophe on a plural, and it should be deleted.
Frequently Asked Questions
It's with an apostrophe is short for it is or it has. Its without one is the possessive, like his or hers. The test is whether you can expand it to it is. If you can, use the apostrophe. Possessive pronouns never take apostrophes, which is why its follows the same pattern as yours and theirs.
APA 7 adds an apostrophe and an s regardless of how the name ends, so Jones's data and Harris's study. Some style guides use only an apostrophe for such names, so check yours. What matters most is consistency across the document.
After the s that is already there. One student's results, several students' results. Form the plural first, then add the apostrophe. For irregular plurals that do not end in s, such as children, add apostrophe s: children's.
No. Write 1990s and 2020s with no apostrophe, because these are plurals rather than possessives. An apostrophe before the s in 1990's would make it possessive, which is almost never what is meant.
No. Several RCTs, three PhDs, two IEPs. Adding an apostrophe here is common and incorrect. The exception is where the result would be genuinely unreadable, which almost never arises with the abbreviations academic writing uses.
Generally no in formal academic work. Write do not rather than don't, it is rather than it's. This removes most contraction apostrophes from your document entirely, which also removes most of the opportunities to get them wrong.
The participants' responses, with the apostrophe after the s, because the responses belong to the participants and participants is plural. Without the apostrophe it is not possessive at all and the sentence usually does not parse.
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