Original	Revisions
'. . . the elderly population, which have special needs . . .'	'. . . the elderly population, who have special needs . . . '
	'. . . the elderly population, which has special needs . . .'

We fired up Microconcord, and while discussion of further points in the student's draft continued, obtained a concordance of population in the context of which and who (one word to the right):

 1  image if not in usage by a whole population which appropriates them as a purely magical partn
 2  anxiety that has emerged among a population which associates years of relative stability with
 3 fferings of the elderly infertile population, which craves nothing so much as a few toddlers t
 4 nd the weary inertia of the human population, which has been depleted by successive waves of e
 5 ission defines the target student population, which has grown from 6,500 to 10,000 in four yea
 6  south-eastern countryside of its population, which has retreated from its razed villages to t
 7 ious effect on the overall prison population which has risen dramatically in the last two year
 8  world community faces: a growing population which has outstripped resources, or soon will. Th
 9 mate number of Luxembourgers in a population which has swelled close to 400,000 with an influx
10 d to happen. That fraction of the population which has even noticed appears to have reacted, s
11 e out in the overcrowded civilian population, which in the main was poverty-stricken, dirty an
12 y multi-racial area with a school population which is one-half black, and the rest white and H
13 try, where the 40 per cent of the population which is ethnic Russian is concentrated. The Kaza
14 red with the UK. An ageing French population, which is concerned about providing for its futur
15 experts. Theoretically, the local population, which is mainly Tibetan, should enjoy rising inc
16 nt and 40 per cent of Macedonia's population, which is becoming increasingly  polarised. Almos
17 69th Parallel, speaks for a local population which knows only central heatin g and triple glaz
18 enix, Arizona, producing a viable population which was then reintroduced to the animals' nativ

19 ned is not the military. It's the population who are angry.' He went on, occasionally smiling,
20 ith a hungry, angry and desperate population who are furious at the double standards they see
21 port among the 55 per cent of the population who are of mixed race (Coloured), the majority of
22 figures - the 4.3 per cent of the population who are full-time veggies added to the 6.5 per ce
23 t the estimated 3 per cent of the population who are dysthymic - and the signs are that this n
24 mi-gods by the 90 per cent of the population who are "bantu peasants ... whose souls are sad a
25 ng nation in the world contains a population who are not capable of deciding who should collec
26  value for the 94 per cent of the population who are basic rate taxpayers with no capital gain
27  been heard by 15 per cent of the population who are seriously obese. But are other fatties al
28 -fifth minority of Britain's male population who are only just beginning to see the sexual ega
29  belong to the 40 per cent of the population who do not even enjoy a formal income). A survey
30 fe. For the 80-90 per cent of the population who don't speak Standard English natively it woul
31 own members of the minority Tutsi population who have escaped the militia-led genocide. When F
32 s of the bottom two-thirds of the population who now live in households with incomes below the
33 defeat the rebels. And to cow the population, who see the rockets defoliate the rain forest wi
34 There is a certain element of the population who seem to spend their time writing or phoning t
35  placed to win the quarter of the population who speak Quechua and other Indian languages, and
36 roduct, to free the eighth of the population who were slaves. But it also killed more American

For a 'collective' noun such as population these data give inescapable evidence for the link between the choice of plural number concord and the personal relative pronoun who/m and that between singular concord and the non-personal which. The student chose which has as a revision.

A reproach sometimes made against data-driven learning is that it 'takes too long' to research the language from data rather than look up the point at issue in a standard work of reference. In that connexion, it might be worth noting that in the course of the consultation it took no more than about a minute out of our discussion to obtain and study a striking demonstration of the connexion between number and relative pronoun choice with collective nouns. As a matter of interest, after the consultation I tried to discover whether this point was covered in any standard grammar of English. After about twenty minutes I tracked it down to p. 771 of A Comprehensive Grammar of the English Language - and suspect that my student would have taken even longer to find it (if she had not given up before then!).