Thank very much to everyone for the info about Figure 1.
I think I've finally figured out how to work out the QALY-based net benefit per individual, that Figure 1 is based on.
I was making a basic error before.
Here is how they work out the QALY-based net benefit per individual:
"Net benefit values were computed for each study participant, defined as:
the value of a QALY
multiplied by the number of QALYs gained
minus the cost (from both healthcare and societal perspectives)."
So to make the calculations, you take the proposed value of a QALY (e.g. £30,000); multiply it by the incremental number of QALYs gained per individual for each therapy, given in Table 6 (this gives you the QALY-based gross total cost benefit per individual for each therapy); and then subtract the QALY-based individual healthcare cost for each therapy (this is calculated by taking the cost per QALY for each therapy, given in Table 6, multiplied by the numbers of QALYs gained for each individual, given in Table 6.)
This gives the QALY-based net benefit values for each individual for each therapy, relative to SMC, which I think Figure 1 is based on. My figures seem to correspond to Figure 1 anyway.
And here are my calculations for three different different QALY values (£30,000, £20,000, £0):
(Negative values are net costs. Positive values are net savings.)
£30,000 QALY value
SMC 0
APT (30000 x 0.0149) [447] - (55235 x 0.0149) [823] = -376 (net cost)
CBT (30000 x 0.0492) [1476] - (18374 x 0.0492) [904] = 572 (net saving)
GET (30000 x 0.0343) [1029] - (23615 x 0.0343) [810] = 219 (net saving)
£20,000 QALY value
SMC 0
APT (20000 x 0.0149) [298] - (55235 x 0.0149) [823] = -525 (net cost)
CBT (20000 x 0.0492) [984] - (18374 x 0.0492) [904] = 80 (net saving)
GET (20000 x 0.0343) [686] - (23615 x 0.0343) [810] = -124 (net cost)
£0 QALY value
SMC 0
APT 0 - 823 = -823 (net cost)
CBT 0 - 904 = -904 (net cost)
GET 0 - 810 = -810 (net cost)
At a proposed QALY value of £20,000 (see above), the net benefit values of CBT and GET are pretty close to zero (i.e. crossing from negative values to positive values). This is near to the (relative) 'zero' value of SMC. So this is why the CBT/GET/SMC lines cross over near £20,000, on Figure 1. This makes me think that I've got these calculations right this time.
Edit: Except, my numbers don't seem exact enough, so there is room for improvement here.