Correct the grammar error in the following sentence:

We performed a quantitative overview of randomized trials which tested β-blockers in myocardial infarction, heart failure, and hypertension.

Highlight for the answer:

We performed a quantitative overview of randomized trials that tested β-blockers in myocardial infarction, heart failure, and hypertension.

Incorrect use of relative pronoun (which vs that) (§7.2.2, Relative Pronouns, pp 317-319 in print). That introduces a phrase that is essential to the meaning of the sentence, and which introduces a phrase that adds more information but is not essential to the meaning. Which should always be preceded by a comma. Another example: “He visited the new hospital, which had been built last year” is correct. However, if there were 2 hospitals and only 1 had been built last year, the sentence would read, “He visited the new hospital that had been built last year.”—Laura King, ELS