Sir Liar ‘Tombstone’ Swansong, ex-CMO-elect, has let it be known that he intends to use his retirement to persuade government to impose a binding minimum price for alcohol, in the hope of curbing alcohol related harm. A figure of 50p per unit sold has been suggested – which would raise the minimum price for a bottle of 12% ABV wine to £4.50, up some 50% on today’s minimum prices.
The government – fearful of loosing votes – has announced that it has no intention of penalising Middle England in the hope of cutting heavy drinking and alcohol related harm. But – ironically – that is what Tombstone and the health tyrants say must happen, if we are to stem the rising tide of alcohol that they say threatens to drown our nation. And Tombstone has a knack of getting his way.
Now, tyrants are not usually first in the queue when it comes to honest use of science, so let us look a little at what they have to say, and see how the science backs up their claim.
The seminal work on the relationship between average values in a population and their relationship to extreme values was done by an epidemiologist called Geoffrey Rose. In a charmingly titled 1985 paper, Sick Individuals and Sick Populations, Rose separated out what he called ‘high risk’ (double-entendre no doubt intended) prevention strategies – strategies that target high risk individuals – from population strategies, where the aim is to shift risk factors in a favourable direction at a population level – and thereby, one hopes, reduce the incidence of disease. Population approaches, opined Rose, had powerful advantages: they were ‘radical’, had ‘large potential’, and were ‘behaviourally appropriate’; and had none of the disadvantages of ‘high-risk’ strategies.
Rose’s 1985 paper was more by way of an introduction to his thinking than a research paper. Five years later he co-authored an even more charmingly titled paper: The population mean predicts the number of deviant individuals – note that sick individuals are now deviant individuals – in which he showed, unsurprisingly, that populations with high mean values of ‘x’ had more individuals with very high values of ‘x’. Populations, for example, with a high average body mass index (BMI) tend to have more obese individuals in them than populations with lower average BMIs.
This is about as earth-shattering as observing that populations with high average height have more very tall individuals in them. It is what we would expect, and indeed the figures – for blood pressure, BMI, alcohol consumption and sodium intake, the variables included in Rose’s 1990 study – bear this out. In a strict, mathematical sense, it is true to say that the population mean predicts the number of deviant individuals. For alcohol, the sums are simple: the percentage of heavy drinkers in a population can indeed be predicted by dividing the mean alcohol consumption (in ml/wk) by ten. A country with a mean weekly alcohol consumption of 150ml will indeed have some 15% of the population classified as heavy drinkers.
Rose then reasoned that, because the population mean predicts the number of deviant individuals, any intervention that reduced the population mean would cause a reduction in the number of deviants: thus, a ten percent reduction in average alcohol consumption would naturally lead to a ten percent reduction in the number of heavy drinkers. And so, in the fullness of time, we have Tombstone and his ilk urging government to hike the price of alcohol, on the grounds that higher priced alcohol means lower average consumption – and so less ‘deviant’ individuals.
But Rose – and all those who have since jumped on the Rose band-wagon – have made a fundamental epidemiological error. Certainly, within the limits of his 1990 study (it is not without other methodological concerns), Rose did show an association between mean population levels and the number of deviants. But – and Rose and Tombstone should know better than to ignore this fundamental epidemiological doctrine: association is not the same as causation.
The data Rose used in his 1990 study was cross-sectional data – that is, data that represents a snapshot in time. Such data can, and did, show, an association, but it can tell us nothing about why the association exists. It could be that a determined cadre of ‘deviant’ individuals – say a subgroup with an unusually marked and invariable tendency to great height – pulls, by a simple mathematical effect, the mean height upwards. Or it may be that higher average height does indeed lead, on average, to more people of exceptional height. Or it may be something altogether different – say that better nutrition in childhood leads to better growth for all – and so both higher average height, and more exceptionally tall people.
Because we cannot know what is causing the association, we cannot make any predictions about how varying one variable – say average height – will affect the other variable – numbers of very tall people. And, by the same token, we cannot know that reducing average alcohol consumption – by a 50% hike in alcohol price for all drinkers – will curb the numbers of heavy drinkers. It is, instead, mere speculation that it might do so. Or it might not. We just don’t know.
Dr No has no doubt that alcohol can be profoundly harmful. But basing wide-sweeping public health policy – policy that will encroach on three quarters of the entire population, the majority of whom are not harmed by alcohol – on mere speculation is bad policy, not to mention bad medicine. It damages professional credibility. Tombstone should lighten up, enjoy a tipple, and do what most people do when they retire – retire.