Students writing their dissertations often confuse their usage of the words that and which with each other. While both are relative pronouns, meaning they introduce an element subordinate to the main clause, the kinds of clauses these pronouns may introduce differ per APA guidelines. APA6 (2010, p. 83) explained:
Clauses introduced by that are called restrictive clauses. These clauses are essential to the meaning of the sentence.
Ex. The cards that worked well in the first experiment were not useful in the second experiment.
Although the use of which is grammatically correct for either restrictive or nonrestrictive clauses, APA prefers to reserve which for nonrestrictive clauses only. This aids in creating writing with precision and clarity.
*Note that commas set off nonrestrictive clauses.
Ex. The cards, which worked well in the first experiment, were not useful in the second experiment.
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