Proper usage of "that" and "which" troubles many writers, as does the question whether or not a comma should be placed before these pronouns. Relative pronouns refer to a previous noun in subordinate clauses. The pronoun "that" introduces a restrictive clause, i.e., a clause that is essential to the meaning of the sentence. In such sentences, there is no comma before the relative pronoun. In the past, the only possible pronoun in such sentences was "that," but nowadays many writers use "which" in place of "that." In principle, this is permissible as long as the comma is omitted if "which" introduces a restrictive clause.
Below are two examples of correct usage of "that" and "which" in restrictive clauses.
^ The books that (or which) provide exercises are particularly helpful. The substances which (or that) performed best in the screening test were those that had a simple chemical structure.
In contrast to the restrictive clause, the nonrestrictive clause is not essential to the meaning of the sentence; it merely provides some additional information. Here, the relative pronoun is always "which," and this must be preceded by a comma.
Let us consider these examples:
^ Some books provide exercises, which is particularly helpful.
The substances with a simple chemical structure performed best in the screening test, which was an unexpected finding.
In essence, "which" that replaces "that" always introduces a restrictive clause supplying essential information. In these sentences, a comma before "which" (or "that") is incorrect. However, if "which" introduces a nonrestrictive clause supplying only incidental information, a comma before "which" is mandatory.
In scientific literature, few words cause more confusion than the term "respectively." Scientific writers of German tongue often use the term in place of the German "respektive." In English, "respektive" is usually a simple "or," and overabundant use of "respectively" frequently obscures the issue.
^ Incorrect: We used TLC and HPLC, respectively, for analyzing the samples.
Correct: We used TLC or HPLC for analyzing the samples.
The word "respectively" indicates a one-to-one correspondence that may not otherwise be apparent between items of two series. If only one series is listed, then the use of "respectively" is incorrect.
^ Incorrect: Incubation times were 2 h, 6 h, and 12 h, respectively.
Correct: Incubation times for experiments 1, 2, and 3 were 2 h, 6 h, and 12 h, respectively.
Confusing sentences containing "respectively" can often be reworded to avoid this troublesome word:
^ Correct but cumbersome: Adverse event rates for the low dose and high dose were below 10% and 20%, respectively.
Better: Adverse event rates were below 10% for the low dose and below 20% for the high dose.
o Only use "respectively" if two series are listed and if there could be ambiguity. Use a comma before "respectively."
Q Exercise 8
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