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This is a continuation of the question at A significant amount of zombies were detected in your city which raised further questions in my attempted answer.

There were purchases. A significant number of them was/were detected.

From [Oxford Dictionaries][1]:

Although the expression ‘a number’ is strictly singular, the phrase ‘a number of’' is used with plural nouns (as what grammarians call a determiner (or determiner)). The verb should therefore be plural:

A number of people are waiting for the bus.

This is not the case with ‘the number’, which is still singular:

The number of people here has increased since this morning.

It can therefore be argued that "them" refers to "purchases". Is "them", "purchases" or "a number" the noun in the second sentence, and why?

Community
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March Ho
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    It is best not to try to analyse 'a number of'; it's a compound quantifier used with count nouns and taking plural agreement (cf the simple quantifier 'many'). – Edwin Ashworth Dec 18 '14 at 11:15
  • possible duplicate of Does "the same number of people" behave as singular or plural? – Drew Dec 18 '14 at 16:45
  • @Drew I can't see the similarity between them, can you clarify why you think it is a duplicate? – March Ho Dec 18 '14 at 16:48
  • I removed my close-as-duplicate vote, but I still think that you are essentially asking the same thing as that other question. You acknowledge that this is a case of a number of and not a case of the number of, but you still ask about them - what it refers to. I can only guess that the underlying question is still why plural is used here. If not, then your question about them is unclear, to me. – Drew Dec 18 '14 at 17:44
  • @Drew This was also brought up when I thought of answering the question (see original answer comments), which is why I made it not about plural, but about which of these is the noun. – March Ho Dec 18 '14 at 17:56
  • The noun is a noun phrase: a significant number of them. And you have already recognized that that takes a plural verb. It's not clear what more you are asking. – Drew Dec 18 '14 at 17:58
  • @Drew the other answer seems to conflict with your definition. I initially thought it was "them" qualified by "a significant number", which I don't really think is that far removed from your answer, but that doesn't work either (see the comment in my answer). Since it is not at all clear what the noun is, can you expand that into an answer, preferably with reasoning and citations? – March Ho Dec 18 '14 at 18:41

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You should not analyse things like “a number of” as prepositional phrases, but rather as premodifiers occupying the determiner slot in a noun phrase.

  • Some purchases were detected.
  • Several purchases were detected.
  • Many purchases were detected.
  • Few purchases were detected.
  • No purchases were detected.
  • A lot of purchases were detected.
  • A number of purchases were detected.
  • Any number of purchases were detected.
  • A handful of purchases were detected.
  • Half a dozen purchases were detected.

In all cases, the head noun of the noun phrase remains purchases, no matter which of those is used. Because the verb must agree in number with its subject’s head, you must use a plural verb in all those cases.

tchrist
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'A significant number' is a stand-in for what can only be imagined as a relatively large number. More importantly, that number of significant size would merit pluralization.

"1 was detected"

"2 were detected"

"A significant number were detected"

"A number [of something] were detected"

As the only number that leads to the singular case is 1, pluralization is the natural choice for unspecified quantities.

To answer your question, the noun is 'number.' The definition of number, per Google:

1. an arithmetical value, expressed by a word, symbol, or figure, representing a particular quantity and used in counting and making calculations.

2. a quantity or amount.

Considering the second meaning of 'number,' the pluralization is much more rational.

EDIT (after comment by Edwin below)

Plural noun usage is explained nicely here.

Relevant quotation:

Note that "the number" is a singular collective noun. "The number of applicants is steadily increasing." "A number," on the other hand, is a plural form: "There are several students in the lobby. A number are here to see the president."

Coty Johnathan Saxman
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    That (your final sentence) doesn't work; quantities and amounts are mass nouns and take singular agreement. The Google definition sense 2 here only works with set expressions (perhaps just the one) such as 'the number of', as they imply. Substitute '267' or 'umpteen' etc as you suggest; 'a significant number' takes plural agreement in spite of 'numbers' not appearing. – Edwin Ashworth Dec 18 '14 at 11:11
  • @Edwin-Ashworth Upvoted. Thank you. I'll clarify the answer. – Coty Johnathan Saxman Dec 18 '14 at 11:14