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.
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”.
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.
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].