Absolute and relative risk
There are two main ways that scientists present risks to the public - absolute risk and relative risk.
Absolute risk is defined as the chance of a person developing a specific disease over a specified time-period. For example, a woman's lifetime absolute risk of breast cancer is one in nine. That is to say, one woman in every nine will develop breast cancer at some point in their lives.
Absolute risk can also be expressed as a percentage (e.g. a woman's absolute risk of breast cancer can be written as 11 per cent instead of one in nine) or as a decimal (one in nine becomes 0.11).
Relative risk is used to compare risk in two different groups of people. For example, in 2002, Cancer Research UK researchers found that women who drank alcohol were at slightly increased risk of developing breast cancer compared with women who didn't drink alcohol. The study showed that, if we call a non-drinker's breast cancer risk '1', then a woman who drinks around two or three units of alcohol per day has a relative risk of 1.13.
What does 'increase in risk' really mean?
Looking at the example above, we can see that women who drink two or three units of alcohol a day have a increase in their chances of developing breast cancer from 1 ('normal' risk) to 1.13, compared with women who drink nothing at all. But how should we react to this figure? Is this a 'big' increase in risk?
Remember that a woman's absolute lifetime risk of breast cancer is 11 per cent. An increase in realtive risk from 1 to 1.13 can also be thought of as a 13 per cent increase in relative risk. But this 13 per cent increase in relative risk is 13 per cent of 11 per cent, or 1.43 per cent.
This means that a woman who drinks two units of alcohol per day has an absolute lifetime risk of 12.43 per cent (because 11 + 1.43 = 12.43), assuming she continues to drink at the same rate. Another way to say this is that women who drink two or three units of alcohol per day have a lifetime risk of breast cancer of one in eight, rather than one in nine (assuming they live to the age of 85).
Usually a scientific paper, news report or press release will make it clear whether it is talking about relative or absolute risk, but it is worth bearing in mind the difference between the two when reading stories about health risk factors.
