Smoking and the risk of developing MND – understanding the statistics

Yesterday we heard that a new study had been published in the Archives of Neurology reporting that people who had (ever) smoked were 1.4 times more likely than people who had never smoked to develop MND. The full details of the research paper are Wang et al Archives of Neurology 2011, 68(2): 207-213.

I’ve been struck by the way that the figures about this study have been reported in the newspapers. So, I decided it was time to get on my soapbox and try and explain the numbers involved.

This study falls into an area of research called epidemiology. The standard way of reporting results is by quoting ‘relative risks’. When two things are equally likely to happen, a relative risk of 1 is assigned. Factors that may make something less likely to happen would reduce the relative risk to less than 1 (eg 0.7). Factors that may something more likely to happen would increase the relative risk to more than 1 (eg 1.4).

A relative risk of 1.4 means that there is a 40% increase in something happening, however, the really important thing to remember is how big the actual number is to start with.

Here is an example, earlier in the month there was a news story about plans to reduce the size of a bar of chocolate, from 140g bar to 120g bar. Which would cause you greater outrage: the fact that the new bar is 14% smaller or that there is 20g less chocolate? Of course, they are both the same, it is just a different way of explaining the reduction in size.

Rather than give you more examples, if you’re interested in this subject, I strongly recommend that you read the Making Sense of Statistics guide produced by Sense about Science.  If you have access to a copy of New Scientist magazine, there is an excellent article on statistics in the 12 February 2011 magazine.

2 thoughts on “Smoking and the risk of developing MND – understanding the statistics

  1. Am I right in assuming that you are putting this research into perspective to prevent any erroneous conclusions and panic? I for one feel that this study is crazy. What ever is the point in telling people that, if they EVER smoked they have an increased (1.4) risk of developing MND? The same applies to the ridiculous statistics surrounding sports people. I was told by a GP, yes, a GP, that my MND was probably due to all the sport that I participated in (I was extremely fit). What of the millions of professional athletes, never mind the leisure marathon runners?Are they all at risk? If everyone panicked about this data we would have a nation of obese, non smoking worriers. There have been so many ‘studies’ over the years creating panic that I take this one with a ‘pinch of salt’!

    • Yes, we wanted to make sure that people didn’t perceive the risk as being much larger than it really is. It’s the media’s interpretation of these studies, rather than the studies themselves, that can sometimes get a little out of hand. Robust evidence that adds to the research community’s knowledge of the possible risk factors associated with MND could lead to increased overall understanding of the disease. However, media reports rarely make clear that the overall risk of getting MND is small and that a lifestyle factor like smoking is only one of many contributory risk factors that must coincide to trigger the disease. Most cases of MND probably develop when a person with a subtle genetic predisposition to the disease then encounters several risk factors over the course of their lives. None of these factors on their own directly cause the disease, which is why most smokers don’t get MND. More information on the causes of MND is available on our website. href=”http://www.mndassociation.org/research/what_causes_mnd/why_me.html”>http://www.mndassociation.org/research/what_causes_mnd/why_me.html

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