Relative pronouns introduce clauses that modify nouns. That and which are two common relative pronouns in scientific writing, with that referring to people, animals, and things, and which referring to animals and things. Importantly, the correct relative pronoun to use depends on whether the clause introduced by the relative pronoun provides essential or non-essential information. Here, we will seek to address the ‘that vs which’ confusion in scientific writing by providing clear examples whilst explaining the function of relative pronouns and restrictive and non-restrictive clauses.

What are relative pronouns?

Relative pronouns (such as who, whom, whose, that, and which) introduce either essential or non-essential clauses to add more information about a noun. These essential or non-essential clauses are commonly known as restrictive/defining and non-restrictive/non-defining clauses. Restrictive clauses add essential information, whilst non-restrictive clauses add information non-essential to the meaning of the sentence.

If the clause being introduced by a relative pronoun is non-restrictive/non-defining, it is proceeded by a comma.

Now we know the basics of relative pronouns and restrictive and non-restrictive clauses, let us address the ‘that vs which’ confusion in scientific writing.

Relative pronouns in scientific writing: that vs which

In scientific writing, meaning depends on essential information being given within a sentence. At the same time, sometimes non-essential information can provide greater context about the nouns being described. That and which have different functions depending on the type of relative clause they are introducing, but they are often mistakenly used interchangeably in scientific writing. Let us look at some specific examples that resolve the ‘that vs which’ confusion.

That in scientific writing

The relative pronoun that refers to people, animals, or things when introducing a relative clause. That is used to introduce restrictive/defining clauses only, meaning it is used when the clause is integral to the sentence’s meaning.

• The electrolytes that were dissolved in water had a high number of freely moving ions.

Here, the relative clause (‘that were dissolved in water’) is restrictive, since it provides essential information. If we were to remove this clause, the meaning of the sentence would be fundamentally changed.

• The electrolytes had a high number of freely moving ions.

By removing the relative pronoun that and the restrictive clause it introduces, the sentence now suggests that all electrolytes in the study had a high amount of freely moving ions, rather than those under specific conditions. That is key to defining specific experimental parameters in studies.

Which in scientific writing

The relative pronoun which refers to animals and things when introducing a relative clause. It is used predominately for introducing non-restrictive clauses, which provide non-essential information.

• The TP53 gene, which was discovered in 1979, is the focus of our current study.

The non-restrictive clause (‘which was discovered in 1979’) adds non-essential information to the sentence. This means it can be removed, and the meaning remains intact.

Even though non-restrictive clauses are non-essential, they are highly useful in scientific writing. Non-restrictive clauses provide extra information about things or methods, giving readers a better idea of historical contexts and study backgrounds.

Typical errors with that vs which

When deciding whether to use that or which, authors can make easily correctible mistakes. Here are some examples of common ‘that vs which’ mistakes and how to correct them.

Missing commas with which

• Incorrect: The formula which we tested last month was successful.
• Correct: The formula, which we tested last month, was successful.

Using which for relative clauses

• Incorrect: The leaf which was from a Spathiphyllum wallisii showed signs of photosynthesis.
• Correct: The leaf that was from a Spathiphyllum wallisii showed signs of photosynthesis.

Using that for non-restrictive clauses

• Incorrect: Our study, that is the first of its kind, aims to help end-users in healthcare.
• Correct: Our study, which is the first of its kind, aims to help end-users in healthcare.

Misusing what instead of that

• Incorrect: The previous study by X uses methodologies what directly inspire our current study.
• Correct: The previous study by X uses methodologies that directly inspire our current study.

Avoiding common grammar mistakes

Using that and which correctly is crucial to making your paper appear assured to reviewers. But difficulties with relative pronouns and English grammar more broadly are still very common, especially for authors writing in English as a second language. This is where professional editing services can help.

At MDPI Author Services, we specialise in helping researchers prepare their manuscripts for publication. Our Academic Editing Service provides subject-specific expertise and in-depth technical refinement during the manuscript preparation phase, helping to improve the scientific content and structure of the paper as the author is guided from pre- to post-publication. This service includes a review of your paper’s terminology. Click here for a free quote.

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