Grammar & Punctuation

Print the original guide from National Institutes of Health Library.


A, an (indefinite). A or an precedes a singular, unspecified, countable noun. Choice depends on the sound of the word it relates to. Use “a” with words that start with a consonant sound; use “an” with words that start with a vowel sound. 

The (definite). Use “the” when writing about a specific entity. Examples: 

  • an hour; a historic occasion; to the fourth decimal; the petri dish on the right 
  • Smith et al. described the experiment as unreproducible. Subsequently, an NIH review board denied the grant renewal. 

A helpful handout on the use of articles is available online from the Perdue Online Writing Lab (OWL) (Search for articles.) 


In “The Science of Scientific Writing,” Gopen and Swan (American Scientist, 1990) write about creating sentences that keep the reader engaged.

Subject-Verb Separation

The reader’s expectation stems from a pressing need for syntactic resolution, fulfilled only by the arrival of the verb. Without the verb, we do not know what the subject is doing, or what the sentence is all about. As a result, the reader focuses attention on the arrival of the verb and resists recognizing anything in the interrupting material as being of primary importance. 

Compare the two examples. Example two is clearer to the reader because of how close the verb (underlined) is to the noun. In addition, the sentence has been edited. 

  • The smallest of the URF’s (URFA6L), a 207-nucleotide (nt) reading frame overlapping out of phase the NH2-terminal portion of the adenosinetriphosphatase (ATPase) subunit 6 gene has been identifiedas the animal equivalent of the recently discovered yeast H+- ATPase subunit 8 gene. 
  • The smallest of the URF’s (URFA6L) has been identifiedas the animal equivalent of the recently discovered yeast H+-ATPase subunit 8 gene

Topic Sentences

“The information that begins a sentence establishes for the reader a perspective for viewing the sentence as a unit: Readers expect a unit of discourse to be a story about whoever shows up first.” 

And, “Readers also expect the material occupying the topic position to provide them with linkage (looking backward) and context.

Parallel Construction

Write sentences that are parallel in structure; create lists parallel in verb tense/noun choice.


  1. Technicians draw blood, analyze samples, and evaluate findings. 
  2. Assign random number to participant
  3. Conduct examination
  4. Order tests 


Apostrophes (’) in possessives and contractions; note:
If two nouns operate as one compound unit and the element of possession is shared by both, only the last noun shows the possessive construction. Grammar and Style at Your Fingertips, p. 128. 

Don’t (do not) use contractions in formal scientific writing. Its is possessive; it’s means it is 

Commas (,) separate clauses/appositives/series; note: the serial comma may or may not be used in American writing and is often not used in British writing—check the publication’s style and be consistent 

Semicolons (;) give pause/connect clauses without conjunctions (conjunctions are words that connect: and, but, or, nor, yet) 

Colons (:) introduce lists or additional details 

Em dash (—) shows emphasis (see example in 2 above) 

En dash (–) indicates a range (e.g., 1996–2006) 

Hyphen (-) connects compound words (e.g., two-fold) and prefixes 

Quotes (“ or ‘) are used for emphasis of a word on first mention, for quoting a source, or for quoting within a quote; American and British English use quotes differently—check the publication’s style and be consistent. 


The committee reported that “Smith and Aldridge stated that the ‘prime’ calculations were fraudulent.” 

Tiding, Ostam, and Fornay’s paper highlighted: cells and cell interactions; tissue-cell regeneration and average time to completion; and cell death. 

The marsupial chased its own tail, similar to the behavior seen in canines. 

From: Chang, E.T. et al. (2007). The burden of liver cancer in Asians and Pacific Islanders in the Greater San Francisco Bay Area, 1990 through 2004. Cancer. 109(10):2100-8. PMCID: PMC2777532. 

  • The slope of each line segment describes the annual percentage change (APC) in the incidence rate, and line segments are connected at “joinpoints” that denote a statistically significant change in trend (P<0.05). 
  • Therefore, we set out to evaluate secular trends in liver cancer incidence rates for APIs combined and Chinese, Japanese, Filipino, Korean, and Vietnamese males and females between 1990 and 2004. 
  • To summarize secular trends graphically, we grouped age-adjusted incidence rates by 5–year periods (1990–1994, 1995–1999, and 2000–2004) in order to stabilize estimates and to examine other changes in the relative incidence and mortality of liver cancer over time. 


Publishers often specify which style to use when writing for their publications. Check the target publication’s instructions for authors for preferred style when formatting body text, in-text citations, and references. Style guides or manuals most often mentioned include those from the American Medical Association (AMA), American Psychological Association (APA), Chicago, Council of Science Editors (CSE), and the International Committee of Medical Journal Editors (ICMJE) publication.

Subject/Verb Agreement

Singular subjects take singular verbs (Dr. Franz is; however, Dr. Franz suggests is singular); plural subjects take plural verbs (findings are; however, findings suggest is the plural form of the verb). 

When the conjunctions “or”, “either…or…”, or “neither…or…” are used, use the form of the verb that agrees with the elements in the sentence (if both elements are singular, use a singular verb; if both are plural, use the plural of the verb). If one element is singular and one element is plural, use the form of the verb that agrees with the element closest to the verb. 


  • The standard paper-based test or the modified online test is used to collect patient data. 
  • Dr. Franz is the past project leader. 
  • Either Dr. Franz or the committee members are responsible for oversight of the project. 


 Many authors are challenged with what verb tense (past, present, future) to use when writing their manuscripts. While it is true that one should generally use the same tense within a paragraph, and within a sentence, there are exceptions. The instructions to authors for the Journal of Clinical Microbiology (JCM) gives a clear explanation of how to use tense in scientific writing:

ASM strongly recommends that for clarity you use the past tense to narrate particular events in the past, including the procedures, observations, and data of the study that you are reporting. Use thepresent tense for your own general conclusions, the conclusions of previous researchers, and generally accepted facts. Thus, most of the abstract, Materials and Methods, and Results will be in the past tense, and most of the introduction and some of the Discussion will be in the present tense. 

Be aware that it may be necessary to vary the tense in a single sentence. For example, it is correct to say “White (30) demonstrated that XYZ cells grow at pH 6.8,” “Figure 2 shows that ABC cells failed to grow at room temperature,” and “Air was removed from the chamber and the mice died, which proves that mice require air.” In reporting statistics and calculations, it is correct to say “The values for the ABC cells are statistically significant, indicating that the drug inhibited …” 

For an in-depth discussion of tense in scientific writing, see p. 191-193 in How to Write and Publish a Scientific Paper, 6th ed. 

Simple Examples of Tense:

  • In this study, we evaluate the effect of temperature variation for freezing cells. (present tense) 
  • In this study, we evaluated the effect of temperature variation for freezing cells. (past tense) 
  • In this study, we will evaluate the effect of temperature variation for freezing cells. (future tense) 


Formality is called for in scientific writing. Contractions are almost never acceptable, but look at The Scientist’s headlines (teasers). Consider the media and the audience if writing outside the bounds of formality. More informal writing may be acceptable for outlets such as Twitter, blogs, or podcasts. Err on the side of professional, factual writing that is concise and engaging.


Sentences that are concise and engage the reader are most often written in active voice. At times, however, passive voice is used to take emphasis off of the person or entity doing the action.

Active Voice 
We conjugated all samples. (passive = All samples were conjugated.)
Participants received a 12-ounce dose or a placebo. (passive = Participants were given a 12-ounce dose or a placebo.)
Passive Voice
Samples were then frozen for 24 hours. (active = We froze samples for 24 hours.)
All patients were contacted to schedule a follow-up evaluation. (active = We contacted all patients to schedule follow-up evaluations.)

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