Punctuation errors rarely change whether an argument is correct, but they do affect how carefully a paper reads — and reviewers, supervisors, and examiners do notice. A few punctuation rules account for most of the errors that show up in academic drafts. Here’s a practical rundown.
The Semicolon: Joining Two Complete Sentences
A semicolon joins two independent clauses (complete sentences) that are closely related in meaning, without a coordinating conjunction: “The sample size was small; the findings should be interpreted with caution.” Using a semicolon where only a comma belongs, or using one to join a sentence fragment, is one of the most common academic punctuation errors.
The Colon: Introducing, Not Separating
A colon introduces something — a list, an explanation, a quotation — and should be preceded by a complete, grammatically independent sentence. “The study tested three conditions: control, low-dose, and high-dose” works because the clause before the colon is complete on its own. “The three conditions were: control, low-dose, and high-dose” is a common error, because the clause before the colon isn’t complete without the list.
The Oxford (Serial) Comma
Most academic style guides, including APA, recommend the Oxford comma — the comma before “and” or “or” in a list of three or more items (“control, low-dose, and high-dose”). It’s not strictly required by all style guides, but including it consistently avoids genuine ambiguity in some sentences, so it’s worth adopting as a default.
Commas With Non-Essential Clauses
A clause that adds extra, non-essential information to a sentence should be set off with commas; a clause that’s essential to identifying what’s being discussed should not be. “The participants, who were recruited from a single hospital, completed a survey” (non-essential, commas used) versus “Participants who completed the survey in under five minutes were excluded” (essential — this clause identifies which participants, so no commas).
Apostrophes in Possessives and Plurals
A frequent source of errors: possessives take an apostrophe (“the author’s argument,” “researchers’ conclusions” for plural possessive), while plain plurals never do (“the 1990s,” not “the 1990’s”; “three studies,” not “three study’s”). Decades, acronyms, and numbers pluralize without an apostrophe.
Hyphens in Compound Modifiers
When two or more words work together as a single modifier before a noun, they’re typically hyphenated: “a well-designed study,” “a peer-reviewed journal.” The same words aren’t hyphenated when they appear after the noun they modify: “the study was well designed.”
Quotation Marks and Surrounding Punctuation
This varies by style guide and is one of the most commonly inconsistent elements in student writing: American conventions (and APA, MLA) place commas and periods inside closing quotation marks, while many other conventions place them outside. Whichever convention your required style guide specifies, apply it consistently throughout the paper.
For a broader look at citation and style conventions, see our style guide comparison or our academic English writing guide. If you’d like a careful proofread, see our editing services or contact us.