The original sentence is:

Post-hoc test performed on the subjects data revealed 6 homogeneous subsets which could be roughly illustrated as follows…

In which I am wondering whether I should insert a comma before "which":

Post-hoc test performed on the subjects data revealed 6 homogeneous subsets, which could be roughly illustrated as follows…

  • This seems like a proofreading question. – user6951 Oct 7 '14 at 15:08
  • So where should I post this question? – Ping Tang Oct 7 '14 at 15:15
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    @CarSmack No, it asks about something very specific. It is not in any sense an off-topic proofreading question. – snailplane Oct 7 '14 at 22:17
  • Great @snailboat then I shall answer it – user6951 Oct 8 '14 at 0:46

First, is that the whole sentence? The ... at the end is used to signify that the sentence continues.

Either way, even if that is the whole sentence, I would put a comma after which. Why?

  1. There is a natural pause before which,
  2. Which introduces a non-restrictive relative clause; these are often preceded by a comma,
  3. The sentence up until which is quite a mouthful to say.

So, now, you have two answers, 1 for the comma, and 1 against. Thus, it is not really a "rule" of linguistics that matters, but making the sentence easy for the reader to read. In one sense, it comes down to opinion, either the people here on stack exchange, or the people over at different English learning websites, or your own. Which I think is most important, as you develop confidence in your English writing skills.

As for me, I do not like commas that much, and they can be overused, but I couldn't see myself writing that sentence without a comma (even if that is the whole sentence). I owe it to my readers to be able to come up for air after the mouthy introductory clause, and a comma is natural before a non-restrictive relative clause, especially one introduced by which.

Here is a good link about comma use in general. Here is a link about restrictive and non-restrictive clauses.

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  • 1
    By feeling, I agree with your answer, which was what I think – Ping Tang Oct 8 '14 at 0:58

My gut sense say that no comma is needed in that sentence. I'm not an expert but commas are used when a pause (by talking or reading) should be inserted. After I wrote this I did some Googling and I'm pretty sure I'm correct in this sole case at least. It's complicated. Read more here: http://www.grammar-monster.com/lessons/which_that_who_comma_or_not.htm

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  • Yeah, from what I've gathered, comma usage can get quite complicated, and much of it is debated by English linguists. – Crazy Eyes Oct 7 '14 at 21:37
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    The comma is naturally and almost always used before which, when which introduces a non-restrictive relative clause. – user6951 Oct 8 '14 at 0:42

The rule is simpler than you thought, if you could remove the which-clause without changing the meaning of the sentence, then you should put a comma before which, or else comma is not used.

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  • Sometimes we place a which-clause between commas and it is necessary for the context. – SovereignSun Mar 16 '17 at 6:04
  • I think this would be a better answer if it answered the question explicitly instead of just stating a general rule. Should there be a comma in the sentence in the question or not, and why or why not? – ColleenV Mar 16 '17 at 12:10

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