Graeme Archer's Diary: Shambo and racial profiling

The City of London crumbles into the borough of Hackney at the interface of Bishopsgate and Shoreditch High Street. Between these two is a tiny handful of properties called Norton Folgate. On Monday evening I passed along this route on the upper deck of the number 26 bus – nothing unusual in that. I was reading Tamburlaine Must Die, a novel about Christopher Marlowe written by Louise Welsh. Just as the bus juddered out of Bishopsgate, I read the following: Where else can a poet live but the bastard sanctuaries? Beggars’ breeding grounds where all are as welcome, or unwelcome, as the other. My lodgings are in a broken-up tenement in Norton Follgate. Well! By chance I found myself passing along the same street that Christopher Marlowe was entering in the novel. This is only the second time I’ve experienced this; the first was half my lifetime ago. I sat in Russell Square one hot, yellow evening in July 1989, reading (with my mouth hanging open) The Swimming Pool Library by Alan Hollinghurst, as the characters made an entrance into the same square. Is this a universal sensation, like deja vu? There must be a Greek word to describe the oddly pleasurable, reality-flickering jolt it serves on you. Russell T. Davies, if you are a Conservative Home columnar regular – and I can imagine nothing else – please take note: there’s a Doctor Who story in there somewhere.

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According to Farming Today & the BBC website (Umbrella Man – I keep my promises) Shambo the Sacred Bull is no longer with us. The Ministry of Death-ray people have succeeded in destroying the animal, despite the angry crowds of Thursday. Let me use the unnecessary passing of the noble bull to illustrate a common fallacy that is relevant for policy decisions about disease screening and terrorism, as well as animal husbandry. Given that Shambo gave a positive test result, what is the probability that he really did have TB? (Or: Given that the suspect fits a particular profile, what is the probability that he is a terrorist? Or: given that a patient gives a positive test result, what is the probability that she has the disease?) Apparently, the probability of the test being positive, if TB is absent, is one in a thousand. But if the background rate of the disease is low, or the false negative rate of the test is high, then the actual probability of Shambo having TB given the result of the assay being positive is not as overwhelming as we were invited to believe. The maths is a simple application of Bayes’ Theorem and the Theorem of Total Probability, both of which you can look up on wikipedia (type in “Bayes theorem”). If we assume a false negative rate of 20% (so 20% of the time the test is negative when the disease is actually present), the graph relating Shambo’s probability of having TB (on the y-axis) as a function of the background rate (on the x-axis) is:

From the graph, we can see that if the assay really does have a tiny false positive rate of one in a thousand then the probability of Shambo having TB is indeed very high, unless the background prevalence of the disease is very low (incidentally – I could not find a reliable estimate of the background rate of bovine TB infection on the web). However, if the false positive rate of the test is higher – around ten per cent – then there is a high chance that he was without TB.

This inductively correct reasoning is germane whenever you hear a government minister calling for universal screening for any disease. The characteristics of the screen and your unconditional probability of having the disease must all be taken into account, before deciding whether the cost outlay is worthwhile – not to mention the stress on the patient induced by a positive diagnosis which may later be found to be faulty. For example, suppose the background rate of a disease is only 1 in 1,000. Even if the false positive and false negative rates of the test are 5/100 and 20/100, respectively, then the probability that YOU have the disease, given that you test positive, would be less than 2%. If you were told “false positive rate is five in a hundred” you would take the test, wouldn’t you? But if you were told “and that means you have a 2% chance of having the disease if you test positive” you might think differently.  Now change “having the disease” to “being a terrorist” and “testing positive in the screen” to “fits the demographic profile”.

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People are always ringing me up and saying, “Graeme, Brown’s ahead in the polls and we came third in two by-elections. What should we do?”  Pausing only to change from my trainers into a pair of good, stout brogues, I always offer the same advice, which has held this party in such good stead for the last ten years or so. Write letters I urge, write critical letters to Influential 1922 Committees, don’t post ‘em, but leak ‘em to the media. People will know you’re putting country before party, and not accuse you of being sore losers. You can be sure that “unelectable rightwing ideologue” won’t appear on your Tombstone, for example. Brushing crumbs from my tweedy legs, I continue Everyone loves a gloom merchant, so if you’re a failed leadership candidate, it’s a really good time to write columns about how we’re all doomed – doomed, d’you hear? No-one will accuse you of being a sore loser, Michael. Either that or offer support to the government over the EU Constitution. No-one will accuse you of being a sore loser, Ken. And if you work for the rightwing press, then just continue the good work of [continues p94].

Wallace Arnold aside, either we pull together and we win, under a moderate, liberal-Tory agenda, or we go on sniping about a past that never was, and we lose. It’s that simple. Who wins – we decide.

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I’ve just realised that Louise Welsh was referring to Hackney as the “bastard sanctuaries”. Cheek! A lot of good people come from Hackney. There’s Ray Winstone. And that bloke from the Fast Show. And Andrew Bo[ deleted to prevent columnar bias – Editor].

Comments

[601]

"On Monday evening I passed along this route on the upper deck of the number 26 bus – nothing unusual in that"

Did not Thacher say something like if you travel on a bus after 30 you should consider yourself a faliure?

[Ken Stevens]

This seems to be a little bit of a mixture between medical statistical analysis (bovine & human) and terrorist profiling. The bovine TB situation is an imappropriate analogy, as we don’t tend to euthanase humans suspected of suffering a particular ailment nor folk suspected statistically of being terrorists.

Perhaps the comparison terrorist-wise is more with the human vaccination programme, i.e. jabbing the majority in hopes of forestalling problems with the minority. Though instances of people catching the disease are comparatively few, the impact on them and their loved ones is great. There is also the point that some folk suffer as a result of the vaccination itself.

So with terrorist profiling: not many people get blown up by bombs, though the personal impact thereof is horrid, and some innocent folk will be adversely affected by preventive profiling – as with vaccination.

However, on the specifics of universal health screening, I agree with the sentiment. Are we supposed to spend our whole lives as the “worried well” by screening for this, that and the next thing.

. ..and if I’ve drawn completely the wrong conclusions from your text, then it’s all your fault for baffling my non-mathematical mind.

[Henry Rogers]

Hi 601

"Did not Thacher say something like if you travel on a bus after 30 you should consider yourself a faliure?"

I've no idea whether Maggie T ever did quote those words or not. But I'm pretty sure that the remark was well known long before she was ever in Parliament, let alone in office. I can certainly remember hearing it attributed to other people, now long forgotten, before she reached Downing Street.

But are there any experts in the trivial here who can tie it down to its original source?

Henry

[Simon Newman]

"Now change “having the disease” to “being a terrorist” and “testing positive in the screen” to “fits the demographic profile”."

Presumably you're opposed to all quarantine measures, then.

[Tequila]

601. Urban Myth I'm afraid. As exposed by Philip Johnston in the Daily Telegraph last November. Oft repeated, never substantiated.

[Sally Roberts]

Graeme I share your anger about Shambo! He was a noble creature and did not deserve to be brought to an end in that way!

[Shambo's Ghost]

It should have been "Cambo" that got the bullet!

[TomTom]

The destroyed the bull and did a post-mortem. It had bovine TB.

With that conclusive evidence what should we infer about dealing with terrorists ?

[DavidTBreaker]

I'm not claiming any knowledge on Bovine TB, but isn't there a farly high background rate from badgers? Apparently 20,000 cattle a year are culled because of it in the UK and they can't all be false positives. How do you decide which are the false positives which can live or do you let them all free and have it spread everywhere?

I tend to think it's fairly posible Shambo was exposed to TB, and the loss of one animal is better than letting it spread. Shambo was, after all, one bull. Apparently in the 1930s around 40% of cows in the UK were infected with with 50,000 new cases of the human infection. Source, Wikipedia.

As for false possitive results in humans, I'd rather that than have a false negative or no test at all and more disease. We must of course keep proportion and not test everyone for everything [the plague would certainly be a bit of a time waste], but I think the current level is fairly light. A private medical system would probably want more, and I'd rather be the 'worried well' than the 'surprised off-guard unwell' or 'suddenly dead' that would be far more common without screening, vaccinations etc. Note the surge in measles since the MMR scare caused a drop in vaccinations. Screening saves lives and money through early treatment and less spread of the infection.

The analogy with terrorism is unusual too. It all comes down to resources. The security services cannot check everyone at the rate they desire, so profiling lets them direct their investigations based on who is more likely to be a terrorist. Do you spend as long screening a hundred year old as a twenty-something, given all the terrorists have been young? In theory all should be checked, both could be terrorists, but resources are limited. We must focus. Profiling for terrorism wouldn't be racial either, but on other factors. It's a sad state of affairs, but we can't blame the security services for focussing resources on those more likely to be terrorists. For any crime with a lack of evidence the police would think what sort of person would do such a thing, and go from that point. With terrorist screening and investigations its just before the main crime has been commited. The impact of screening should be minor though, as most of it should be done by the secret service in secret.

I'd rather get annoyed at being stopped than get blown up or see others killed by terrorists. I'd rather have occassional sreenings than public health epidemics. And I'd rather false possitive TB cattle get culled than risk have it spread.

[601]

Thank-you Tequila and Henry Rogers.

[sjm]

Should having a different religion from that established in the country in which you live absolve you from observing certain laws?

My answer would be no. I don't know about the accuracy of the bovine TB test, and I'm far too tired to read your technical analysis, but every damn cattle farmer in the country has to abide by these laws, and I see no reason why the monks don't have to.

And of course, wonderful things come from Hackney .... like me (altho' a long time ago).

[malcolm]

,Either we pull together and we win',never a truer word spoken Graeme.

[Angelo Basu]

sjm- a difference might be that cattle farmers are breeding cattle for food, transporting them to markets/abattoirs where they may come into contact with other animals and the food chain. I query whether the same public health considerations must apply where an animal is a pampered and revered pet which will not be slaughtered or eaten.

[page]

With blogs like this around I don't even need website anymore. I can just visit here and see all the latest happenings in the world.