From Furious of Feltham

[lathany]

One of this evening's news headlines is a story about a mother whose conviction for the murder of her children was overturned. As a result of this, the BBC website produced a page commenting on the suitability of one of the witnesses, namely Sir Roy Meadow.

Now, Sir Roy belongs to a class of people who really annoy me. Being responsible for convicting an innocent woman ? No (well, actually yes, but not specifically what I'm complaining about). It is because he is a non-statistician claiming to be an expert on statistics.

This happens... far, far too often. Doctors are particularly guilty of it because the use of their own data in the field of medical statistics makes a vocal minority think they know how to interpret all the results. But the lack of statistical training really, really shows. And it gives those of us who actually work in the field a bad name.

Grrrrr.

Comments

Stats Blindness

[bateleur.livejournal.com]

Agreed.

I mentioned the statistical fallacy condign was laughing at in his law school blog (see here (http://www.threeyearsofhell.com/archive/000341.html) if you missed it) to a group of work colleagues.

I got a mixture of blank looks and general snorting about the evils of homophobia, but was surprised to note that the underlying fallacy was not immediately obvious to anyone there.

Re: Stats Blindness

[bateleur.livejournal.com]

(Footnote: This is actually a response to dr_bob's post below... he must've spotted a typo and redone it !)

Re: Stats Blindness

[verlaine.livejournal.com]

Not sure I understand myself - why should it be permissible to discriminate against homosexuals as a higher risk group if discriminating against higher-risk black donors (presumably a "racist" act) is out of bounds?

Re: Stats Blindness

[bateleur.livejournal.com]

That's not the matter under discussion. The question is not whether the treatment of homosexuals in this case is right or wrong (which is politics, not statistics) - it's: Can you spot why the statistical argument given is bogus ?

If someone has a serious point to make, they damage their case significantly by trying to use a technical argument and getting it wrong.

Re: Stats Blindness

[onebyone.livejournal.com]

But you said So we're looking at the population "all heterosexuals".

I don't think that covers it. The statistical claim compares homosexuals with African-Americans, and is indeed bogus as stated.

However, let's look deeper. Accepting the stats quoted, and using the fact that 13% of Americans are black plus the estimate that 2-5% of the population are practising homosexuals we see that yes indeed, "homosexual blood" is at least (42/5) / (33/95) = 24 times more "dangerous" than "straight blood". However, "African-American blood" is still roughly (54/13) / (33/95) = 12 times more dangerous than average "straight blood" (including straight African-Americans).

"Dangerous" here means "likely to be from a donor who is a recent case of HIV infection".

So, assuming all the things that need to be assumed about the independence of being gay, being black, and infection rates in the various intersections, I think it's reasonable to question the criteria by which "high-risk" groups are defined.

However, someone else on the law blog has questioned whether infection mechanisms are similar in the ways required. They say that in fact most black HIV infections would be excluded from giving blood for other reasons, or just don't give blood. In the absence of that extra information suggesting that "black blood" is safer than it looks, there is a case to answer.

Re: Stats Blindness

[onebyone.livejournal.com]

Sorry, I meant "times as dangerous" in each case, not "times more dangerous".

Re: Stats Blindness

[http://users.livejournal.com/_alanna/]

Yep, I think there's a further issue that both homosexuals and black Africans would be less likely to make it through our blood donation system than heterosexual whites for reasons completely unrelated to HIV, e.g. Hepatitis, malaria etc.

Re: Stats Blindness

[onebyone.livejournal.com]

True. I don't think either the original argument or my calculation concludes the case, but both illustrate the point that much more information is required before we can accept the NHS position that "homosexuals are too dangerous a group to take blood from".

Re: Stats Blindness

[bateleur.livejournal.com]

Although you're out by a factor of 2-ish since not all homosexuals are male. And then there's the fact that 2-5% is a Kinsey-style figure, so in fact includes bisexuals.

None of which invalidates your basic observation that it's a "Where do we draw the line" question... Still, if I wanted to touch on the politics as well as the stats I'd throw in socio-economic group as a factor and then there'd be something to argue over !

Re: Stats Blindness

[verlaine.livejournal.com]

As far as I could see the comparison was between homosexuals (a high percentage of AIDS cases despite coming from a numerically small subsection of the population) and black people (a high percentage of AIDS cases despite coming from a numerically small subsection of the population). The fact that someone on a cursory glance might think that there is no greater prevalence of AIDS in the homosexual than the heterosexual population is fairly irrelevant if that's not the issue under discussion.

I didn't see any claim or implied claim that a given homosexual in the catchment area is scarcely more likely than a given heterosexual to have AIDS, but maybe I didn't investigate the relevant documents closely enough. That's the claim that would get my hackles up.