The twenty most common errors in college writing were identified by Andrea Lunsford and Robert Connors in research examining thousands of student essays in the late 1980s. Explanations of the errors have been adapted from The Everyday Writer by Lunsford and Connors (New York: St. Martin’s Press, 1997, pages 11-14).

1. Missing comma after an introductory element
Many sentences begin with one or more introductory elements—clauses, phrases, or single words that lead into the main body of the sentence. To show where an introductory element ends and the main part of the sentence begins (thus helping your readers move more easily through the sentence), insert a comma after the introductory element. If the sentence includes more than one introductory element, use a comma after each one.

Incorrect: Although they carry out the same overall functions prokaryotic and eukaryotic cells differ in their structural organization.

Correct:   Although they carry out the same overall functions, prokaryotic and eukaryotic cells differ in their structural organization.

2. Vague pronoun reference
Readers should be able to easily identify a pronoun’s antecedent (that is, the specific word or phrase to which the pronoun refers). If the antecedent is unclear (either because there is more than one possible pronoun reference or because the word to which the pronoun refers is implied but not actually stated in the text), you need to make the reference more explicit.

Incorrect: Within the central cavity of the micelle, molecules could accumulate and chemical reactions could take place; these were pivotal in the evolution of life on Earth.

Correct:   Within the central cavity of the micelle, molecules could accumulate and chemical reactions could take place; these processes were pivotal in the evolution of life on Earth.

3. Missing comma in a compound sentence
When two independent clauses (clauses that can stand alone as separate sentences) are joined by a coordinating conjunction such as and, but, so, yet, nor, or for, precede the conjunction with a comma to signal a pause between the two parts of the sentence. The pause gives pace to the sentence and helps prevent sentence misreadings.

Incorrect: The archaea and the bacteria are unique among organisms in having prokaryotic cells and these two organisms evolved from a common progenitor cell along distinct lines of evolution.

Correct:   The archaea and the bacteria are unique among organisms in having prokaryotic cells, and these two organisms evolved from a common progenitor cell along distinct lines of evolution.

Note: Be sure to distinguish between compound sentences, as defined above, and sentences that have two or more predicate phrases but only one subject. Sentences with a compound predicate do not take a comma between the two parts of the predicate. For example:

Incorrect: The archaea and the bacteria have prokaryotic cells, and evolved from a common progenitor cell.

Correct:   The archaea and the bacteria have prokaryotic cells and evolved from a common progenitor cell.

4. Wrong word
Most instances of this error arise from confusion or carelessness regarding homonyms or other words that either sound similar or have similar meanings. Wrong-word errors may also result from overly casual use of a thesaurus.( Be sure to consult a dictionary if you’re not sure of the exact meaning of a word you find in a thesaurus.)

Incorrect: There cells are usually surrounded by protective walls composited of peptidoglycon and teichoic acid.

Correct:   Their cells are usually surrounded by protective walls composed of peptidoglycon and teichoic acid.

5. Missing comma(s) with a non-restrictive element
Non-restrictive elements are words, phrases, and clauses that provide relevant information in a sentence but are not essential to the sentence’s basic meaning. Non-restrictive elements in a sentence should be set off (on both sides) with commas. In the following example, the non-restrictive element is underlined. The intended message of this sentence is that all archaea are both structurally similar to bacteria and molecularly related to eukaryotes.

Incorrect: The archaea similar to bacteria in terms of structural organization are also closely related to eukaryotes when examined on a molecular basis.

Correct:   The archaea, similar to bacteria in terms of structural organization, are also closely related to eukaryotes when examined on a molecular basis.

6. Wrong or missing verb ending (-s or –es, -d or –ed)
In spoken English, we sometimes either omit verb endings altogether or pronounce them inaudibly, but standard written English requires their use even when other information in the sentence implies these endings. Make sure that subject and verb agree (plural or singular) and that you are using the correct verb tense. (See also #10.)

Incorrect: No cell wall separate the virus from its surrounds.

Correct:   No cell wall separates the virus from its surrounds.

7. Wrong or missing prepositions
A sentence’s meaning may change depending on the preposition you use in conjunction with a verb or as part of a prepositional phrase. For example, the act of comparing something to something else involves looking for similarities among the two items; the act of comparing something with something else means registering both similarities and differences between the items. The acts of meeting at or in an intersection have different implications, as do the acts of talking around, about, or through an issue.

Incorrect: Some organisms that scientists once considered algae have been reclassified based by their cellular structure and organization.

Correct:   Some organisms that scientists once considered algae have been reclassified based on their cellular structure and organization.

8. Comma splice
Inexperienced writers sometimes combine two or more independent clauses (clauses that are capable of standing independently as separate sentences) into a single sentence by inserting a comma between the clauses. This error can be corrected in several ways: (1) by separating the clauses into stand-alone sentences; (2) by replacing the comma with a semicolon; (3) by following the comma with a coordinating conjunction such as and, but, so, yet, nor, or for; or (4) by rewriting the sentence to subordinate or eliminate one of the independent clauses.

Incorrect: We observed a patient with German measles, we found objective evidence that this patient had been infected with the rubella virus.

Correct:   We observed a patient with German measles. We found objective evidence that this patient had been infected with the rubella virus.
or
We observed a patient with German measles; we found objective evidence that this person had been infected with the rubella virus.
or
We observed a patient with German measles, and we found objective evidence that this patient had been infected with the rubella virus.
or
When we observed a patient with German measles, we found objective evidence that this patient had been infected with the rubella virus.

9. Missing or misplaced possessive apostrophe
Possessive nouns (nouns that indicate possession of something else) generally have an apostrophe-s ending if singular (e.g. microbiologist’s or Ross’s) or just an apostrophe if plural (e.g. microbiologists’ or Rosses’). Possessive personal pronouns (e.g. hers, his, ours, theirs, yours, and its), however, do not take apostrophes.

Incorrect: The microscopes initial capabilities were insufficient for complex studies of bacteria.

Correct:   The microscope’s initial capabilities were insufficient for complex studies of bacteria.

10. Unnecessary shift in tense
Unless you have a clear reason for doing otherwise, use the same tense for all verbs both within and across sentences.

Incorrect: He covered one portion of meat with a loose-knit cloth; this step prevents the flies from reaching that portion.

Correct:   He covered one portion of meat with a loose-knit cloth; this step prevented the flies from reaching that portion.

11. Unnecessary shift in pronoun
When you use a pronoun reference in a sentence, make sure that you use the same pronoun for all subsequent references in the sentence. The most common example of pronoun shifting is the shift between one and I, you, or we.

Incorrect: Looking only at these data, we might be tempted to agree with the study’s conclusions. And yet, if you succumbed to that temptation, you’d be seriously off track.

Correct:   Looking only at these data, we might be tempted to agree with the study’s conclusions. And yet, if we succumbed to that temptation, we’d be seriously off track.

12. Sentence fragment
A sentence fragment is an incomplete portion of a sentence that is punctuated as a full sentence. To correct this error, incorporate the fragment into the preceding or subsequent sentence, or rewrite the fragment as an independent clause. In some types of writing (for example, in fiction and journalistic writing), authors may incorporate fragments as a stylistic choice. In academic writing, however, the use of fragments is far less common.

Incorrect: That brief but eye-opening visit to the research laboratory was a turning point in my career. Leading to my decision to pursue virus research.

Correct:   That brief but eye-opening visit to the research laboratory was a turning point in my career, for it led to my decision to pursue virus research.
or
That brief but eye-opening visit to the research laboratory was a turning point in my life. The experience led to my decision to pursue virus research.

13. Wrong tense or verb form
The verbs in a sentence must clearly communicate whether the condition or action being reported in the sentence is happening in the past, present, or future, whether the action is definite or conditional, and so on. Be on the alert for irregular verbs (verbs that don’t follow the standard pattern for indicating tense) that have been treated as regular verbs.

Incorrect: I recognized that even viruses had their parasites, and I had named these parasites “defective interfering virus particles.”

Correct:   I recognized that even viruses had their parasites, and I named these parasites “defective interfering virus particles.”

14. Lack of subject–verb agreement
The verb form used in a sentence may vary depending on whether the subject is singular or plural and whether the sentence is written in first-, second-, or third person. When you are looking for subject–verb agreement errors, be mindful that the noun closest to the verb may not be the subject.

Incorrect: Light microscopy use visible or ultraviolet light to illuminate an object.

Correct:   Light microscopy uses visible or ultraviolet light to illuminate an object.

When the subject consists of two or more nouns connected by and, the subject is generally plural. However, when all parts of the subject refer to the same thing or person, the subject is considered singular (see the second example following).

Correct:   Reflection and refraction are properties of light.

Correct:   At next week’s meeting, our instructor and mentor intends to discuss her forthcoming study and our role as research assistants.

When the subject consists of two or more nouns connected by or or nor, verb agreement should be with the part of the subject that’s closest to the verb.

Correct:   Neither the original research design nor the adjustments we made to that design as the study progressed fully account for our study’s unexpected results.

15. Missing comma in a series
To avoid potential sentence misreadings, always insert a comma between the last two items in a three-or-more-item list.

Incorrect: Gram, acid-fast and endospore staining are differential staining procedures.

Correct:   Gram, acid-fast, and endospore staining are differential staining procedures.

16. Lack of agreement between pronoun and antecedent
When you use a pronoun (such as you, him, she, their, it) in place of the noun to which it refers (called the pronoun’s antecedent), make sure the pronoun agrees with its antecedent in both gender and number.

Incorrect: All of the organisms we examined under the microscope had its own unique characteristics.

Correct:   All of the organisms we examined under the microscope had their own unique characteristics.

Note that the words each, every, one, and (often) either and neither are singular and therefore require singular pronoun references. Also, singular noun antecedents joined by or or nor require a singular pronoun.

Incorrect: Each of the organisms we examined under the microscope had their own unique characteristics.

Correct:   Each of the organisms we examined under the microscope had its own unique characteristics.

Incorrect: Neither the Bacteria nor the Archaea have a nucleus.

Correct:   Neither the Bacteria nor the Archaea has a nucleus

Some noun antecedents (for example, “sociologist” or “researcher”) are non-gender-specific; that is, these words refer to both males and females. When you use this type of antecedent in its singular form, your pronoun references to the antecedent must acknowledge both genders. You have several options for establishing pronoun agreement with non-gender-specific nouns: (1) For every pronoun reference, use “him and her, “she and he,” etc. Some writers find this awkward and choose instead to alternate between male and female pronoun references either paragraph by paragraph or section by section. (2) You can change the sentence wording to make the antecedent (and therefore the pronoun) plural. (3) You can simply get rid of the pronoun. All three solutions are illustrated here.

Incorrect: Each microbiologist felt compelled to purchase his own microscope.

Correct:   Each microbiologist felt compelled to purchase her or his own microscope.
or
All of the microbiologists felt compelled to purchase their own microscopes.
or
Each microbiologist felt compelled to purchase a microscope.

17. Unnecessary comma(s) with a restrictive element
Restrictive elements are words, phrases, and clauses that are essential to the basic meaning of the sentence. Do not set restrictive elements apart from the rest of the sentence with commas. In the following example, “of pure cultures” is essential to the sentence’s meaning. (Isolation as a general concept is not exclusively concerned with microorganism samples.)

Incorrect: Isolation, of pure cultures, involves separating samples of microorganisms into individual cells.

Correct:   Isolation of pure cultures involves separating samples of microorganisms into individual cells.

18. Fused sentence
Fused, or run-on, sentences occur when clauses that could stand alone are joined with no linking words or punctuation. Correct this error in one of four ways: (1) Separate the clauses into two (or more) sentences; (2) insert a semicolon between the clauses; (3) insert a comma followed by a coordinating conjunction (e.g. and, but, so, yet, nor, for) between the clauses; or (4) rewrite the sentence to subordinate or eliminate one of the independent clauses.

Incorrect: Light microscopy uses visible light to illuminate an object in microbiology, this tool is particularly useful for determining gross morphology of bacterial cells.

Correct:   Light microscopy uses visible light to illuminate an object. In microbiology, this tool is particularly useful for determining gross morphology of bacterial cells.
or
Light microscopy uses visible light to illuminate an object; in microbiology, this tool is particularly useful for determining gross morphology of bacterial cells.
or
Light microscopy uses visible light to illuminate an object, and in microbiology, this tool is particularly useful for determining gross morphology of bacterial cells.
or
Light microscopy, which uses visible light to illuminate an object, is particularly useful in microbiology for determining gross morphology of bacterial cells.

19. Misplaced or dangling modifier
To prevent sentence misreadings, place modifying words and phrases as close as possible to the word or words they modify.

Incorrect: I have thought that saving lives and helping humankind would be a wonderful way to spend the rest of my life since the age of seven.

Correct:   Since the age of seven, I have thought that saving lives and helping humankind would be a wonderful way to spend the rest of my life.

20. Its/It’s confusion
Its indicates possession; it’s is a contracted form of it is or it has.

Incorrect: The work we’ve done with this microorganism shows that its possible to establish in vitro the necessary conditions to support it’s optimal growth.

Correct:   The work we’ve done with this microorganism shows that it’s possible to establish in vitro the necessary conditions to support its optimal growth.

Some of these sample sentences were adapted from Ronald M. Atlas’s Principles of Microbiology (Dubuque: Wm. C. Brown, 1997). Handout prepared by Taira L. Wilson, Oregon State University, 1998, and revised by Tracy Ann Robinson. With thanks to Dr. Linda Bruslind, OSU Department of Microbiology, for her content review.