Are the Brits really fatter than other Europeans?

Lots of press reports in the last couple of days on how UK women are the fattest in Europe, for example in the Daily Mail and on the BBC News website. I'm still in Berlin, and it was in the papers here too. The tabloid-style Berliner Kurier went with the headline "Man, they are fat, man", while the N24 news service went with "British and Maltese are the fattest Europeans". But is it another dodgy league table?

Well, yes, though for different reasons from those we've looked at here or here. And probably we shouldn't be blaming the reporters, because they made a tolerably good job of reproducing data in the press release from Eurostat, the EU statistics office, that they were reporting on. It does really say that 23.9% of UK women are obese (body mass index (BMI) over 30), and this is indeed the highest percentage out of all 19 countries they looked at. The figure for men is not much lower, 22.1%, and this is indeed the second highest (a bit less than Malta)...

...except there's some quite important small print in the press release. First, it points out that the data aren't for the whole of the UK at all, but just for England. The BBC article mentioned this, correctly pointing out that things aren't any better in terms of obesity in the other countries of the UK either. Then, Eurostat says in the same footnote that for England, "adult" means 16 and over, whereas it's 18 or over in all the other countries. Actually that's not going to make much difference to the percentages either, but it made me wonder why it should be different. The press release says that the data are from the European Health Interview Survey (EHIS), published by Eurostat, and that the EHIS aims to measure various things across the EU "on a harmonised basis". Using different age groupings in different countries doesn't sound very harmonised to me. What's going on?

There's another footnote in the press release, about the EHIS. All that does is provide a link to another article on the Eurostat site - but this one helpfully explains that the English data don't come from the EHIS survey at all, they come from the Health Survey for England. (The Italian data aren't from EHIS either.)

Now the Health Survey for England isn't like many routine health surveys. Participants aren't just asked questions about their health and so on. If they agree, the interviewer actually measures their height and weight, and indeed a nurse comes and takes several other physical measurements of various kinds. It's these measurements that are used to work out the body mass index, and hence to provide data on how many people are overweight or obese.

In contrast, the EHIS survey (and indeed the survey that was used to provided data for Italy) did not take actual measurements, but simply asked people their height and weight.

This matters. The scales used in the English survey don't lie - they might not be utterly accurate in every way, but compared to asking people their weight and height, there are unlikely to be major biases. People do not always know exactly how much they weigh, and even if they do, they might tell the interviewer a rather smaller weight, or perhaps add a centimetre or two to their height. That would bring down the person's body mass index, and if it happens systematically, it will introduce a bias into the obesity figures. Furthermore, it's likely that the amount of bias will be different for men and women, and for different age groups, and possibly for different countries too. And, without having data from surveys using physical measurements in all the other countries, we don't even know how big these biases are likely to be.

So the figures for the other 18 countries aren't really comparable with the English figures. There are biases, but we don't know how big. The Eurostat data say, for instance, that a very alarming 16.6% of young English women (aged 16-24) are obese, far greater than the figures (actually for ages 18-24) for all the other countries listed. (Malta is next with 10.7%, and most of the others are well below 5%.) Is this because English young women are really so much fatter, on average, than women elsewhere in Europe, or are young women more likely than, say, old men to knock a kilo or two off when an interviewer asks their weight?

This isn't really very comforting to us Brits, though. The percentages that are overweight are still scarily high, and the knowledge that, possibly, we're not top of this league after all, doesn't bring our percentages down. It's the other countries' figures that are likely to be subject to bias, not ours. Maybe the Maltese and the Latvians really are heavier than us, on average - but if so, that's because they are heavier than the survey said they were, not because we are any lighter. (And I really shouldn't conclude anything at all from my observation that the people I see on the street here in Berlin do seem less likely to be really large than the people I see on the street back in the UK. That observation, is likely to be even more biased!)

Moral: it's worth reading the small print in press releases, even if they come from a respectable statistical agency. Things might not be as simple as they seem.