Comma before "which" in the following sentence? [duplicate]

I can't seem to determine whether or not the second half of the sentence below is restrictive or non-restrictive and thus, whether or not a comma is needed? While it doesn't seem necessary, it does seem to be rather important.

However, a previous history of drug use was found to be strong predictor of program success, which suggests that interventions should be put in place sooner rather than later.

I've spent the last little bit going over restrictive and non-restrictive clauses, as well as dependent and independent clauses separated by coordinating conjunctions and am a tad confused at this point. Is this a case where it is a case of personal preference?

Any help is appreciated.

Many thanks!

Solution 1:

There are other ways to classify relative clauses than restrictive and non-restrictive. The Cambridge Grammar of the English Language (p1034), for example, lists integrated, supplementary, cleft, and fused relative constructions.

Your sentence has an example of what the CGEL would call a supplementary relative clause. It states:

The supplementary relative...permits a much wider range of antecedents, as is evident from examples such as:

• i. Pat is afraid of snakes, which I'm sure Kim is too.
• ii. Pat is afraid of snakes, which doesn't surprise me at all.

The antecedents for which here are an AdjP in [i] and a whole clause in [ii]... .

Your sentence is an example of the [ii] type. The "which suggests that interventions should be put in place sooner than later" relative clause has the whole "a previous history of drug use was found to be strong predictor of program success" main clause as its antecedent.

Supplementary clauses must be marked off with a comma. The CGEL says:

The information expressed in this type of relative is presented as supplementary, and this is reflected in the fact that the relative clause is characteristically marked off prosodically or by punctuation from the rest.

Solution 2:

You have asked a good question. But at first I want to clarify the defining (restrictive) relative clause and non-defining (continuative) relative clause with two examples :

Now compare the following two sentences :

(A) The place where I went to school is a wonderful town. (B) Edinburgh, where I went to university, is a beautiful city.

In (A) We can’t say “The place is a wonderful town” … why? Because we don’t know which place it is. This means that it is essential to put the complete defining clause :The place "where I went to school" is a wonderful town. (Here the quoted part is a Defining Relative Clause because it defines or describes its antecedent "The place". This clause clearly defined which place it was. Therefore, we DO NOT USE COMMAS here.

In B it is possible to say “Edinburgh is a beautiful city” and “where I went to university” is an extra information that grammatically is not necessary. Both parts are grammatically complete sentences (grammatically this type of Relative Clause is called a non-defining Relative Clause, because it does NOT define the subject, it only adds information, this type is written with COMMAS at the beginning and end).

Therefore, your posted sentence contains a non-defining relative clause : However, a previous history of drug use ....... predictor of program success, which ( = and it ) suggests that interventions should be put in place sooner than later.

Here the relative clause is adding extra information about the program success, and here which stands for 'and' it'. It's therefore a non-defining relative clause and a comma should be used before 'which'.

Hope, it's now as clear as daylight! Thank you, everyone.