Of course it won’t! The reason we don’t believe that it will is because we can’t see any reason why it should. We don’t believe things unless we have a good reason.

But I would expect people who owned expensive cars to be, on average, healthier than people who own old, inexpensive cars or no car at all. Why? Because people who own expensive cars are rich. And rich people, on average, are healthier than poor people. Rich people can afford a better diet and to live in places with cleaner air. They tend to be better educated and so can understand the benefits of a healthy life-style. But if you change your car, it won’t cause all of these things to suddenly happen to you!

So there is likely to be an association between having an expensive car and enjoying good health. But one is not the cause of the other.

Now suppose that you read, or see on the television, that “low 25-hydroxyvitamin D levels are associated with a higher prevalence of peripheral arterial disease”. Would you immediately believe that a high intake of this vitamin is good for you? Most people would. But this statement alone provides no evidence for this belief. It is making a statement about association – just like the association between ownership of expensive cars and good health. It tells us nothing about cause. There may be something that influences levels of 25-hydroxyvitamine D that also influences the likelihood of having peripheral heart disease. I don’t know – I’m not an expert on heart disease.

But associations of this kind are commonly taken by journalists, advertisers and the people who believe them, to be the same thing as identifying a cause. Worse still, based on this assumption, people may waste money on worthless dietary supplements to prevent an illness when there is no real evidence that it will do so.

 

Related posts

16.8 Predictions

16.3 Scientific proof

Follow-up posts

16.15 Science education
16.36 Good and bad
16.42 Keep it simple
16.43 Test before you predict
16.46 The placebo effect
17.1 It’s obvious