Saturday, 23 July 2011

Public health and unwarranted conclusions

I read a fair number of articles. This is a paraphrase of one but I have seen similar several times over.
In conclusion our results show that exposure X is associated with significantly increased chance of outcome Y. Public health recommendations/ government agencies should reduce/ ban exposure X.
(This is assuming outcome Y is bad, a converse argument could be made if exposure X is thought to be good.)

This is frustrating for several reasons.

1. The word significantly usually means statistically significant. That is the authors are confident that the association they have found is a real one, not a chance one. For various reasons I think many findings that are claimed to be real are actually chance, so I may not be convinced the statistics justify the conclusion. But assuming the statistics do justify it, the significance relates to the degree of confidence in the result, not the size of the result. The phrasing "significantly increased chance" sounds like the size of the association is strong. It may be minor. A risk ratio of 1.003 (1.002–1.004, p <0.001) is statistically very significant but not functionally significant. Even a risk ratio of 3 (i.e. you are 3 times more likely to develop outcome Y) may be irrelevant if the outcome is extremely rare. Does it really matter if you increase your risk from 1 in a million to 3 in a million?

2. Association is not causation. Yes it may be a real effect, and it may be a relevant one, but it still may just be an association. We need to establish causation. Addressing a problem if it is causative may not resolve it. Addressing an association that is not causative definitely will not resolve it. And it could potentially worsen it. We need studies that show definite causation. Then we need studies that show intervention to reduce exposure X actually reduces outcome Y.

3. Nothing in the research relates to public policy. The study does not show that the policy was enacted and was effective. Even convincing knowledge that reducing outcome Y by preventing public exposure to X does not imply anything should be done by the state about X. Should the state ban hang-gliding because it is associated with increased mortality? Should it make every vice illegal because of detrimental effects on self? And there are further question about enforcing a ban. What about the monetary cost considerations? What about liberty? Will the unintended consequences be worse than the problem? Perhaps all that is warranted is education. For example the government can mandate labelling without banning a substance.

I am not arguing against any public policy. It just seems that socialism is so embedded in some people's psyche that new information to them logically implies government intervention.

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