Friday, 9 September 2011

This week's nomination

I'm a fan of Auckland's Stats Department's "Bad Stat of the week" contest. I won their inaugural one by nominating John Pagani. I would have won another one but for lack of competition; alas.

Today's entry? Jennie Connor's trashing of a rather nicely conducted study on the health benefits of moderate drinking. Let's start with the study.

Qi Sun and a team including Eric Rimm found that moderate alcohol consumption correlates with good aging and health outcomes in middle-aged nurses in the US: the Nurses' Health Study. They follow a set of over a hundred thousand female registered nurses, starting in 1976 and surveyed every two years subsequently. It was set up to look at the long-term effects of oral contraceptive use but has been used in lots of other studies since.

They averaged reported alcohol intake in the 1980 and 1984 follow-up studies to get a measure of mid-life alcohol consumption, then checked to see whether alcohol consumption at mid-life increased the likelihood of successful aging, defined as being free of major chronic diseases and having no major cognitive or physical impairment and no mental illness.

To avoid the "sick quitter" confound, they dropped from the sample any participants who had:

  • Chronic diseases at baseline
  • Those diagnosed with alcohol dependence or chronic liver cirrhosis
  • Those reporting a significant reduction in alcohol use within 10 years before baseline
So, we shouldn't have former drinkers who quit because of poor health included in the reference group.

Next, they worried about confounding through other health behaviours. They controlled for a wide range of dietary, health, physical activity variables and life history of smoking. When you have observations every two years on a large group of women over a thirty year period, you can do a lot. But, you still can't completely correct for things you can't observe.

What can you do? You can check to see how much of the effect is reduced when you add in all the health behaviours for which you can control. If the effect drops substantially, it's likely there are other things for which you can't control that are correlated with the things for which you can control and might further reduce things. If the effect doesn't move much, it's not likely that other health behaviours are driving things.

Table 2, above, gives the relevant comparisons. Compared to non-drinkers, the odds ratio for those consuming 1.5-3 standard drinks per day was 1.26 in a model adjusting only for age and 1.28 in a model adjusting for all the health and activity correlates. It's exceedingly unlikely that unobserved health behaviours would attenuate the effect if correcting for observed health behaviours accentuates the effect.

They also run things within a cohort restricted to never-smokers. The pattern remains, though statistical significance is lost (you don't get a lot of never-smokers among nurses in a cohort that starts in 1976).

Ok. so what does Connor bang on about then?

The idea that moderate drinking may be good for health is well entrenched, but has been increasingly questioned, including by New Zealand epidemiologists Professors Jennie Connor and Rod Jackson, who challenged the theory in Britain's Lancet journal in 2005.
Professor Connor, of Otago University, told the Science Media Centre the Harvard study added nothing to the many similar studies that were "unreliable for answering questions about the health effects of drinking because of their design".
The supposed health benefit might be due to differences in lifestyle - other than drinking - that were associated with being a low-risk drinker.
"It may be true that women who drink one drink a day are healthier than others, but we do not know if it has anything to do with the alcohol, as these women are not the same as others in a variety of ways."
There was no scientific justification for the promotion of alcohol as health-enhancing for any sub-group of the population.
"The potential for harm is great, and the potential for good is unknown."

What had Connor and Jackson warned about in the Lancet? Two things that Sun, Rimm et al do a pretty good job in ruling out: confounding of never-drinkers with former drinkers, and unobserved health behaviours.

Jennie: if it's unobserved health correlates that are doing the job, why the heck does the relationship get stronger once they corrected for other health behaviours? If some underlying "healthy type" is driving things, that'll correlate with the other health behaviours that we can observe and will reduce the effect of alcohol on successful aging, not increase it. In other specifications, there are small reductions in the odds ratio in the multivariate model as compared to the bivariate one. But the reduction is small, like from 1.43 to 1.35. If throwing in a kitchen sink of health behaviours that are likely to correlate with the unobserved health behaviours reduces the effect by less than twenty percent, it's pretty unlikely that full controls would reduce the odds ratio to 1 or less.

And so I'm nominating Jennie Connor for the bad stat of the week contest for her trashing of what seems a pretty solid study.

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