In research papers, many badly written sentences can be fixed by changing a non-parallel structure to a parallel one. A parallel construction is made up of grammatically equal elements, like a series of noun phrases or a series of adjectives. Although sentences do not have to be parallel to be grammatically correct, a parallel structure is often easier to read.
In this blog article, I explain how to fix common errors in parallel structure to make your research paper more readable.
In most scientific papers, a test variable is compared with a control to determine its effect on a particular outcome. These important comparisons are clearer if the elements being compared match grammatically. For example, you can compare a noun with a noun, but you shouldn’t compare a noun with an adverb. Consider the following example:
Chemotherapy but not changing the patient’s diet improved overall survival in our study group (Incorrect).
This sentence is difficult to read because the comparison is not parallel. Take a look at the words on either side of the comparative phrase (but not) – a noun (chemotherapy) is being compared with a participle phrase (changing the patient’s diet). We can correct this by changing the participle phrase to a noun:
Chemotherapy but not dietary therapy improved overall survival in our study group (Correct)
or by changing the noun to a participle phrase:
Subjecting the patient to chemotherapy but not changing the patient’s diet improved overall survival in our study group (Correct).
Words and phrases can be linked together in a sentence using correlative conjunctions. Examples include either/or, neither/nor, not/but, not only/but also, and both/and. These conjunction pairs can add emphasis, discuss different options, or express a contradiction. Importantly, the elements joined by correlative conjunctions must be equal. For example:
Either patients were treated with the drug or the placebo, but not both (Incorrect).
This sentence is wrong because either and or join a subject-verb combination (patients were treated with the drug) and a noun (the placebo). We can fix this sentence by changing the subject-verb combination to a noun:
Patients were treated with either the drug or the placebo, but not both (Correct).
This sentence is parallel because either and or join two nouns (the drug and the placebo). If you read the two examples again, you will see that the parallel sentence is easier to understand.
Another common error in research writing is to use these conjunction pairs to compare more than two things. For example:
We measured either heart rate, blood sugar, or blood pressure in each patient (Incorrect).
Here, we are comparing three things, so the correlative conjunction either/or is not correct. We can fix this by removing either:
We measured heart rate, blood sugar, or blood pressure in each patient (Correct).
A parallel series
A series is a list of three or more items in a sentence. For clarity, each element of a series should be grammatically equal. For example:
Suitable treatments are surgery, occupational therapy, and with drugs (Incorrect).
You probably noticed that this sentence doesn’t sound right. The problem is it’s not parallel – surgery and occupational therapy are nouns, and with drugs is a prepositional phrase. We can fix this by changing the prepositional phrase to a noun:
Suitable treatments include surgery, occupational therapy, and drugs (Correct).
All three series elements are nouns, so the sentence is now parallel and much easier to read. Let’s take a look at another example:
We treated the patients with chemotherapy, curative surgery, and analysed the patient outcomes (Incorrect).
Again, this sentence is not easy to read; the third element (analysed the patient outcomes) is a separate predicate and does not belong to the series.
We treated the patients with chemotherapy and curative surgery, and analysed the patient outcomes (Correct).
This sentence is now parallel; the two elements in the series are nouns and the predicate has been separated from the series with a comma and a coordinating conjunction.
A preposition indicates where something is or when something happened (e.g., in, on, under, over, after, before, at). Prepositional phrases include a preposition and its object. In a parallel series of prepositional phrases, the preposition must be repeated at the start of each element. For example:
Blood samples were collected in the operating theatre, postanaesthesia care unit, and during follow-up (Incorrect).
This sentence is incorrect because there is no preposition before postanaesthesia care unit. This can be easily fixed by adding the missing preposition:
Blood samples were collected in the operating theatre, in the postanaesthesia care unit, and during follow-up (Correct).
But if the same preposition introduces each element, then the preposition does not need to be repeated. For example:
Patients were contacted by telephone, by email, and by post
can be shortened by removing the last two prepositions:
Patients were contacted by telephone, email, and post.
For a series of verb phrases to be parallel, the same verb should introduce each element. For example:
These data show reduced postoperative complications, more patient satisfaction, faster recovery, and confirm improved graft survival (Incorrect).
This sentence is wrong because the final element (improved graft survival) is introduced with a different verb (confirm). Consider how that final element now reads (These data show confirm improved graft survival) and the solution becomes clear – we have to get rid of the second verb:
These data show reduced postoperative complications, improved patient satisfaction, faster recovery, and improved graft survival (Correct).
Or we can put the final element into a separate sentence for added emphasis:
These data show reduced postoperative complications, improved patient satisfaction, and faster recovery. Improved graft survival was also confirmed (Correct).
Parallel structure is also important in bulleted lists. Take a look at this example:
The goals for this procedure include the following:
The last point in the list is not parallel because it starts with a gerund (reducing) while the other points in the list start with infinitives (to close and to minimize). The error can be fixed by starting the last point with an infinitive:
or by starting the first two points with a gerund:
In some sentences, the parallel construction is deliberately avoided. These are called elliptical constructions and they exclude information that is implied by the rest of the construction to improve readability. For example, consider the following two sentences:
Two studies used a paper-based tool, four used an electronic approach, and two did not report on this.
Two studies used a paper-based tool, four studies used an electronic approach, and two studies did not report on this.
Which sentence is easier to read? The first one, because it is not necessary to repeat studies. The reader infers that we are talking about the number of studies in each element.
A common mistake is to re-insert the eliminated word somewhere in an elliptical series. For example:
Two studies used a paper-based tool, four used an electronic approach, and two studies did not report on this
may cause the reader to stop and wonder why studies is used again in the final element. Were we not talking about the number of studies all along? Remember the golden rule: make life easy for your reader.
Get the right balance
You do not have to be a grammar expert to spot errors in parallelism. Non-parallel sentences are unbalanced and usually difficult to understand, unless you are deliberately using an elliptical construction to improve readability. The tips outlined in this article will help you to write parallel sentences to make your meaning clear in your next research paper.
Balancing sentences can be tricky
Claire Bacon is a former research scientist with professional qualifications in copy editing and medical editing. She edits manuscripts for non-native English-speaking scientists and works as a copy editor for a medical journal.
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