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Run-ons and Fragments
Writing Tip Wednesdays March 11, 2026
This writing tip is from Luis M. Luque, a writer-editor in the Division of Communication Science and Services. Luis has been working at CDC since 2012. Before that he served on active duty for 20 years as a journalist/mass communication specialist in the U.S. Navy.
Sentence fragments are usually not permitted in formal or professional writing, but sometimes people don’t realize when they’ve written a sentence fragment or a run-on sentence. For anyone allergic to grammar, I’ll keep this as short as possible. A sentence is a complete thought that consists of a subject and a verb. Consider this simple example: “John ran.” “John” is the noun and subject. “Ran” is the verb.
For imperative sentences, where you are directly telling the reader what to do, you can use an implied subject. For example, “You, go” and “Go” mean the same thing. “Go” is the shortest sentence in English. In formal or professional writing, imperative sentences are the only time to use an implied subject.
OK, so what’s a sentence fragment? If your string of words has a subject and a verb, it’s probably a sentence. If not, it’s a fragment, which reads as though it’s missing crucial information.
Examples
- The tall one, the one wearing the striped tie. What about the tall one? Did he do something? (Missing verb)
- Ran to the park. Well, who ran to the park? (Missing subject/noun)
- After the staff meeting. Who did what after the meeting? (Missing both subject/noun and verb)
Please note that sentence fragments are permitted in bulleted lists and parenthetical statements. According to the Chicago Manual of Style, we should not put a period after a sentence fragment in a bulleted list. This contradicts the outdated CDC Style Guide. Do not mix sentence fragments and full sentences in a single bulleted list or in a single parenthetical statement.
Run-on sentences are multiple complete sentences that aren’t separated.
Example
- The dog chased the cat it ran away.
That’s two complete thoughts: (1) The dog chased the cat. (2) The cat ran away. How do you separate run-ons? You can use periods. You can use coordinating conjunctions like “and,” “but,” and “or.” You can also use semicolons to join independent clauses, but this should be used rarely. If a period works just as well as a semicolon, use a period.
Examples
- The dog chased the cat. The cat ran away. (Period)
- The dog chased the cat, and the cat ran away. (“And” is a coordinating conjunction. Note that we always use a comma before a coordinating conjunction leading into an independent clause.)
- The dog chased the cat; the cat ran away. (Semicolon)
Do not separate run-on sentences with commas. That’s the dreaded comma splice we heard about in elementary school.
Example
- The dog chased the cat, it ran away. To fix it, you could use “so” after the comma.
Note, however, that separating your ideas in these ways can make your writing seem choppy. A good way to make your writing flow better is by using words that show the relationship between your ideas, even within the same sentence.
Examples
- Because the bus was late, I was late for school. (Subordinating conjunction “Because”)
- The bus was late, so I was late for school. (Coordinating conjunction “so”)
- Although I love pizza, I try not to eat it every day. (Subordinating conjunction “Although”)