Saturday, June 2, 2012

Newsflash: everyone being healthier reduces deaths

People like me often complain about the accuracy of scientific journalism, and the selective nature of the reports we see in the media; indeed, there's plenty to complain about.  However it's perfectly possible to misrepresent a story simply by choosing to place a particular spin upon it, usually one which makes the story seem more political than the original research.  It's easy to blame the newspapers for this, but often the problem is compounded by academic press releases, which are themselves designed to catch the eye of media outlets.

A good example of this comes from a story this week about alcohol intake, which reports that, essentially, if everyone drank only one half of one unit of alcohol per day (i.e. 5 ml of pure alcohol per day), this would save thousands of lives per year.  Many news outlets picked this up, and it even found its way onto Radio 4's The News Quiz, where the idea that people should only drink about 125 ml of beer per day was thoroughly ridiculed.  The Daily Mail's headline, for example, began "Don't drink more than a quater of a pint a day", followed by "Oxford study claims slashing the official alcohol limit would save 4,500 lives a year."

You can get the original paper here (open access, happily), and I recommend a look to see the contrast with what you might have been exposed to in the news.  If you're not a scientist, or imagine that scientific papers must be completely incomprehensible then I think you'll be pleasantly surprised; it's perfectly readable, largely free from unnecessary jargon and technical terms, and unlike many reports you might encounter in the worlds of business and politics, admirably concise (7 pages, including tables and figures).

Notice the difference between the newspaper headlines and the title of the paper: "What is the optimal level of population alcohol consumption for chronic disease prevention in England?"  Aside from being rather less exciting sounding, note that this makes no mention of strategies for individuals, nor about telling people what to do.

The results summary tells us that "reducing the median consumption of alcohol to 5 ml per day would avert or delay 4,579 (2,544 to 6,590) deaths per year" [note to the authors, rounding is your friend].  There are three things about this sentence which strike me.  First, unlike the news stories, there is a 95% credible interval associated with the estimate of the number of "lives saved"; it could be as low as 2,500, or as high as 6,600, which is a pretty wide margin.  This doesn't need further statistical explanation here, suffice to say that there is a good deal of uncertainty about how much effect alcohol has upon different aspects of people's health.  Second, we're talking about the median alcohol intake, not everyone's intake, which is not the same; under this scenario half the population drink 5 ml per day or more, and half drink 5 ml or less.  Lastly, it mentions deaths "averted or delayed".

Now you might wonder what averting our own death might mean, as opposed to delaying it, which is seems more realistic.  Sadly the paper doesn't explain precisely, but here's my best guess: the paper is based upon a series of simulations under different scenarios of how much people drink, using estimates of the effects of alcohol on various diseases.  Then they count the number of deaths each year under the various scenarios; 1,000 deaths averted or delayed simply means that the number of deaths counted in the simulation was 1,000 fewer than under the status quo.  Now, from an individual point of view, it could just be that you die the next year instead; or you could live another 20.  The study doesn't consider this, because their interest is from a public health perspective.

Risk and reward

This is perfectly reasonable from the government's point of view, and in a purely economic sense: if someone lives for a year longer before they get liver cirrhosis or some other unpleasant chronic condition, then that's one more year before the public purse has to shell out for your treatment.  Standard economic theory applies a discount to a cost in the future as compared to something you have to pay for now.  In particular, it only requires a pretty small proportion of people to live a bit longer for this to turn into a significant amount of cash saved.  And as a bonus your citizens live happier, longer lives, etc.

From an individual's perspective, however, the chance of benefiting from this reduced risk of death may be extremely small, and when presented with the facts it would be perfectly rational to decide that you'd rather enjoy a few drinks (or a bacon sandwich) than have a slightly longer life expectancy.

In any case the articles in the news imply that scientists are telling us all to drink less, rather than just reporting that drinking less would might be good for us.  In particular the Daily Mail's article is pretty explicit in saying that "scientists say" that we should do such and such, and even claims that "changing the guidance" would save the 4,500 lives, rather than people actually drinking less.  The paper itself says explicitly otherwise [my emphasis].
The recommendations and public messages...that would be required to achieve this...level of consumption are beyond the scope of this work...  Public health behavioural recommendations should ideally be based on the best available evidence for optimising population health outcomes.

Who should we shout at?

Now to my favourite part: assigning blame.  In this case, the BMJ's own press release is at least partly at fault, and reads pretty similarly to the BBC article.  The credible interval has been removed, even though it is a very important measure of uncertainty inherent in the study, and, in this humble statistician's view, is perfectly capable of being understood by the non-scientific general public.  That the level of uncertainty in scientific studies tends not to be reported contributes to the general impression that science always provides precise answers to questions; this causes problems when people read about studies with conflicting results, and leads to the contradictory impression that scientists don't know what they're talking about, and that one may as well ignore them.  Naturally, neither of these things are true.

As an example of how this translates to media-world, the Mail says [my emphasis again]
The new advice flies in the face of previous studies, which have shown that drinking alcohol in moderation reduces the risk of dying from heart disease. 
This is completely untrue.  The paper is itself a meta-analysis of other results combined with a simulation, and does not involve a new study in the sense of following drinkers and seeing what they die from.  The model used takes account of the fact that alcohol appears to be protective against heart disease, but finds this to be outweighed by other risks.

I'm uncertain as to who writes these sorts of press releases.  There's plenty of pressure on scientists to get their research noticed by the general public, and the lead scientist of this study is quoted in some detail by the Mail.  Getting this kind of attention depends upon newspapers and other media outlets picking up these stories, which in turn depends upon those outlets persuading their customers that the story is worth reading about.  Implying that it's telling them all to stop drinking is a good way to grab someone's attention.

Some people take the view that it's better for scientists to hide behind a wall of solid fact, especially on issues such as climate change and evolution, because the idea that some scientists disagree on any aspect (however trivial in comparison with the might of the whole idea) might suggest that it's somehow equally valid to assume climate change is not down to carbon dioxide, or to believe in intelligent design.  I think that this is likely to be highly counterproductive, and that it would be far better for scientists and the media to give a more realistic idea of what science, and being a scientist, is really like.  The BMJ improving their press releases would be a good place to start.


  1. Just noticed that David Spiegelhalter beat me to this one:

    He talks about the paper more directly, and I disagree with his tone a little: I still think "What is the optimal amount of alcohol to consume from a public health point of view?" is a perfectly reasonable epidemiological question to ask, but agree that the paper and its press release lent themselves to being interpreted the way they were.

    1. Further update - some other useful commentary here: