Grammatical number of "a specified number of people" [duplicate]

Solution 1:

Firstly, it is counter-productive to analyse 'of weak events' here in the same way as one would analyse say 'of weak events' in 'Properties of weak events'. The of belongs rather with a [specified] number of, and a number of is a compound (multi-word) quantifier. Contrast The numbers of locomotives over 50 years old are getting hard to read and need cleaning (eg locomotives number 11031, 11032, 11033...) (prepositional phrase) with The number of locomotives over 50 years old is very low (we've only six left now) (compound quantifier usage). See http://www.myenglishpages.com/site_php_files/grammar-lesson-quantifiers.php .

Secondly, as Reg suggests, the situation is complicated by what the meaning of the statement is (though 'a number' is singular, it is almost certainly referring to more than one event, and this is taken into consideration in 'logical concord' - see http://grammartips.homestead.com/number.html for a discussion of this particular quantifier. Different quantifiers are handled differently, and not always predictably.)

And thirdly,

We will not stop operating until a specified number of weak events have been detected.

is probably intended.