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Restrictive and Nonrestrictive Clauses

Set nonrestrictive clauses (extra information) off with commas and introduce them with “which.”

The samples, which James analyzed, had been stored for 2 weeks.

(Only 1 set of samples is being discussed; therefore, the clause “which James analyzed” is not necessary for defining the samples.)

Do not place commas around restrictive clauses (necessary information) and introduce them with “that.”

The samples that James analyzed had been stored for 2 weeks.

(More than 1 set of samples is being discussed. The clause “that James analyzed” defines the noun “samples,” telling which samples were analyzed.)

Subject–Verb Agreement

See also Prepositional phrases, verb agreement in.

Use a singular verb with a singular subject and a plural verb with a plural subject, even if a plural phrase follows the subject.

Our analysis of all patients with E. coli infections shows…

Collective nouns (e.g., number, total, staff, faculty) take a singular verb when the noun is regarded as a group and a plural verb when individual members of the group are emphasized.  A good rule of thumb is to use a singular verb when the article is “the” and a plural verb with “a.”

The number of patients was unclear.

A number of patients were quarantined. [patients were quarantined individually]

The number (total, group) reported is 25.

A number (total, group) of persons are…

Seventy-eight percent is the largest percentage reported.

The Centers for Disease Control and Prevention is...

In a noncount noun in a prepositional phrase with "none," the verb plurality is determined by the object of the preposition.

None of the blood was...

None of the patients are participating.


dialyze blood, not animals or humans

transfuse blood, not animals or humans

A quantity indicated by a unit of measure is singular, even when the units are spelled out (ASM).

Twelve milliliters was injected.

...10 mg was added

...5 mL was injected

...220 cpm of radioactivity was detected


Nonabsolute or informal units

...10 drops were added

Do not allow a mathematical symbol to stand for the main verb in a clause (ASM).

Incorrect: When p <0.005...

Correct:  When p was <0.005...

Avoid using nouns as verbs.

Incorrect: was electrophoresed

Correct: underwent electrophoresis

Incorrect: The patient was biopsied.

Correct: A biopsy was performed.

Prepositional phrases, verb agreement in

Mass nouns (e.g., number) are sometimes followed by a prepositional phrase that includes a plural noun (e.g., of patients). In such cases, the article that precedes the mass noun signals whether the mass noun or the number of the noun in the prepositional phrase controls the number of the verb.

If a definite article (e.g., the) precedes the mass noun, then the mass noun controls, and a singular verb typically is used:

The number of patients who may become ill is substantial.

If an indefinite article (e.g., a or an) precedes the mass noun, then number of the noun in the prepositional phrase controls, and a plural verb typically is used:

A small percentage of the patients have complications.


See also Apostrophes.

Year indications are not possessive.

1960s, not 1960's

Organization names vary:

American Medical Writers Association

Department of Veterans Affairs

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