HIV trends

Given this blog’s recent focus on things claiming unconvincingly to be surveys, you must be expecting a post on the statistic that 20% of HIV-positive gay men in Auckland don’t know they’re infected.

It’s obviously going to be hard to get an accurate estimate, since we don’t have a citywide list of gay men in Auckland. We don’t know what proportion of the population is gay; in fact, we wouldn’t even be able to get consensus on what the definition would be.

The approach used by the Otago researchers was to visit places like bars, and events like Big Gay Out.   This gives a reasonably well-defined sampling frame — the sample isn’t from all gay men in Auckland, but we do know who was targetted.   About 50% of the people they approached agreed to fill in a questionnaire, and 80% of those gave a saliva sample that was subsequently tested.   It’s not perfect, but it’s the best you are likely to be able to do in practice; a sharp contrast with the bogus polls on the farm sales, where simple random-digit dialing for a sample of ten people would have been better.

The final numbers supporting the conclusion are small:  68 men were HIV-positive; 15 of them were not diagnosed. The  ‘1 in 15’ figure could be as low as 1 in 20 or as high as 1 in 12, and that’s before you start worrying about bias from non-responders being different. A comparison to other surveys of this kind in NZ and in other parts of the world is still sensible, and the research paper says the infection rate is lower than in most places, but the proportion who don’t know they are infected is higher.

There’s a lot of research currently on ways to sample from populations that can’t be reached effectively by random-digit dialling, but where there are social links between members: jazz musicians, injecting drug users, homeless people.  The general approach is to get people to recruit each other, and the difficult part is to try to correct for the bias this causes, but it’s not clear that the current methods actually work.

Incidentally, the NZ Herald report contains the strange paragraph

The researchers compared respondents’ self-reported HIV test history with their saliva result to find 1.3 per cent of HIV positive men did not know they were infected.

which initially doesn’t seem to make any sense and contradicts the headline.  Most of the problem is the usual inattention to denominators:  take 15/1068, to get the proportion of testable samples that were HIV positive and undiagnosed, and you get close to 1.4%.  That is 1.4% of sampled men were HIV positive and didn’t know it. Confusing P(A and B) with P(A|B) is a bit unusual — usually the Herald confuses P(A|B) with P(B|A).

The 1.3% figure actually appears in the research paper, and it seems to be a problem of premature rounding:  round the proportion HIV positive to 6.5% and the proportion of those undiagnosed to 20%, and you get 6.5%×20%=1.3%.