Friday, August 27, 2010

Relative risk and absolute risk

Statistics can be deceiving if one is not careful. We learn of things that double our risk of developing this or that disease or increase it by say 25 or 50%. We also learn of things that decrease our risk of developing a disease by a certain percentage. It may sound very important to have a risk doubled or reduced to half but unless we know how much it originally was, can we really tell how much the change is? If you get a job and your boss tells you that your salary will be double the salary of Mohammed you immediately ask: How much is the salary of Mohammed? We do not do the same when we learn about doubling or halving the risk of a certain condition.  We do not bother to know how much the original risk was. Unless we translate a relative risk (i.e. a risk expressed as a ratio of another risk) into an absolute risk (i.e. the chance of developing an event regardless of how it compares with another risk) we cannot judge its magnitude and importance. For example, if you know that smoking increases your risk of developing a cardiovascular event in the next 10 years by say 50% and your absolute risk is already 20%, the increase is 10% which is important and worth avoiding smoking (leaving aside other harms of smoking). On the other hand if someone tells you that using your mobile phone increases your chance of developing an acoustic nerve tumour by 100% and your statistical chance of developing this disease (i.e. your absolute risk) is 1/100,000 then using the mobile phone will increase it to 2/100,000 i.e. an increase of 1/100,000. Most people will consider this increase, even if it is true, insufficient to make them stop using mobile phones. I can mention other examples of things that decrease the risk and the same thing applies.
We should always remember that it is the absolute risk that counts.

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