Grammar is important for effective communication. Without correct grammar, your ideas and arguments will not come across clearly in your research paper and you will confuse your reader. Even worse, your good work may be discredited.
Subject-verb agreement is a simple (and important) grammar rule that is often violated in research writing. In this blog post, I describe some common causes of subject-verb disagreement and explain how to avoid them.
Verbs must agree with their subjects. This means if you have one subject then your verb must be singular, and if you have more than one subject then your verb must be plural:
Blood and saliva was collected from the patient upon admission to the ward (Incorrect)
Blood and saliva were collected from the patient upon admission to the ward (Correct).
Nice and simple so far. A common mistake in research writing is to complicate sentences by separating the subject from the verb with long intervening phrases. This often leads to subject-verb disagreement because it is no longer clear which subject the verb should agree with. For example:
Blood and saliva, two of many bodily fluid samples analysed by our clinical assistant, was collected from the patient upon admission to the ward (Incorrect).
The problem here is that the verb (was collected) no longer agrees with the subject (blood and saliva). The correct verb is were collected. The best solution is to keep the subject and verb together and to use two separate sentences if necessary:
Blood and saliva were collected from the patient upon admission to the ward. These were two of many bodily fluid samples analysed by our clinical assistant (Correct).
Brackets cause the same problem. Any information enclosed in brackets should be ignored when deciding which verb to use. For example:
Neurological monitoring (particularly intercranial pressure and partial oxygen pressure) show whether the brain is vulnerable to hypoxic damage (Incorrect)
Neurological monitoring (particularly intercranial pressure and partial oxygen pressure) shows whether the brain is vulnerable to hypoxic damage (Correct).
The subject of this sentence (neurological monitoring) is singular so the verb (shows) should be singular too. Intercranial pressure and partial oxygen pressure is parenthetical information and does not affect the verb.
Some conjunctions – like in addition to, together with, and as well as – do not make the subjects they join together plural. Using these conjunctions to replace and is a common mistake in research writing and usually leads to subject-verb disagreement. For example:
Blood as well as saliva were collected from the patient upon admission to the ward (Incorrect)
Blood as well as saliva was collected from the patient upon admission to the ward (Correct).
And be careful with or and nor – these conjunctions offer an alternative; they do not join two things together. This means that the verb is not automatically plural. So which subject should the verb agree with? Take a look at this sentence:
Fibroblasts or astrocytes grow in this culture medium (Correct).
In this example, both subjects (fibroblasts and astrocytes) are plural so we use a plural verb (grow not grows).
If one subject is plural and one is singular, then the verb should agree with the subject it is closer to:
Aggression or suicidal behaviours was reported (Incorrect)
Aggression or suicidal behaviours were reported (Correct)
Suicidal behaviours or aggression was reported (Correct).
Singular or plural?
Some plural nouns are commonly mistaken for singular ones in research writing. These include criteria, bacteria, media, and data. For example:
The only exclusion criteria was previous gastric surgery (Incorrect)
The culture media was changed every 48 hours (Incorrect, if we are talking about one type of culture medium)
The only exclusion criterion was previous gastric surgery (Correct)
The culture medium was changed every 48 hours (Correct).
Whether or not a singular verb can be used with data remains controversial. Many medical journals prefer data to have a plural verb, while others argue that data can have a singular verb if it is being used as a non-count noun (i.e., a noun whose items cannot be counted, such as information). Check the guidelines of your chosen journal if you are not sure.
Similarly, some nouns thought to be plural because they end in –s, are, in fact, singular, e.g., measles, genetics, economics:
Measles are a leading cause of vaccine-preventable death (Incorrect)
Measles is a leading cause of vaccine-preventable death (Correct).
A pronoun is a word that takes the place of a noun (e.g., I, we, it, they, each, every). They can cause problems with subject-verb agreement because they often sound plural but are, in fact, singular:
- Each and every are always singular. For example:
Each of the participants were debriefed after the study (Incorrect)
Each of the participants was debriefed after the study (Correct).
- Either and neither are also singular:
Neither of the treatments were successful (Incorrect)
Neither of the treatments was successful (Correct).
- None and all are not always singular – they can be plural depending on whether the antecedent is a countable or non-countable noun. For example:
All the patients were satisfied (patients is a countable noun, so the verb is plural)
None of the information was helpful (information is a non-countable noun, so the verb is singular).
Changes in subject number
Elliptical constructions exclude information that is inferred by the rest of the construction. The missing information may include a second verb, for example:
The patients were tested and their data analysed.
Here, it is not necessary to say “their data were analysed” because we can infer this from the rest of the sentence. However, if the number of subjects changes in a sentence, this type of construction is no longer correct. For example:
The patient was diagnosed with cancer and chemotherapy sessions started (Incorrect).
It is no longer possible to use an elliptical construction because one subject (the patient) is singular and the other (chemotherapy sessions) is plural. Therefore, it is necessary to include a second verb that agrees with the second subject:
The patient was diagnosed with cancer and chemotherapy sessions were started (Correct).
Or we can change the second subject (chemotherapy sessions) to a non-countable noun and retain the elliptical construction:
The patient was diagnosed with cancer and chemotherapy started (Correct).
Other common mistakes
The word of also causes problems with subject-verb agreement in research writing. Often, the verb is mistakenly changed to agree with what comes after of – but the subject always comes before of. For example:
The list of potential study participants were given to the study co-ordinator (Incorrect)
The list of potential study participants was given to the study co-ordinator (Correct).
In this example, the subject is list not potential study participants so the verb (was) must be singular.
It can also be tricky to get subject-verb agreement correct when describing numbers. The number and the total are singular and a number of and a total of are plural, as follows:
The total was 100 recruited participants.
A total of 100 participants were recruited.
Subject-verb agreement is important in a sentence and is one of the easiest grammar rules to understand. But it is often violated in research writing. By avoiding the common pitfalls outlined in this article, you can make sure that your subjects always agree with their verbs and that your meaning is clear.
Make sure that your subjects agree with your verbs and all will be well
Claire Bacon is a former research scientist with professional qualifications in copyediting and medical editing. She edits scientific research papers and teaches courses on scientific writing.