As a general rule, a pronoun should refer clearly to a single, explicitly stated antecedent and should agree with that antecedent in number. In my scientific editing, I encounter one particular violation of this rule in nearly every paper that crosses my desk. Here’s an example:
The ZnO peaks for the samples that were dried for 5 days are smaller than that for the sample that was dried immediately.
The singular demonstrative pronoun “that” must have a singular antecedent, but the sentence contains no singular nouns. The author clearly meant “that” to refer to “ZnO peak,” which is implied but not explicitly stated. You might be inclined to leave this sentence unrevised, thinking that everyone will know what you mean. However, what about the following sentence?