What are your chances of getting cancer?
January 09 2011
Cancer and other cardiovascular diseases are pretty common these days . Usually the chance/risk of you getting /preventing/ treating cancer/other diseases is usually expressed in percentages since we are not certain about their outcome.
But sometimes these percentages can be deceiving.
Which drug will you take to lower the risk of cancer?
- Drug A can lower the risk of dying from cancer by 25%
- Drug B can lower the risk of dying from cancer by 0.1%
Which drug will you take? Obviously, Drug A.
In fact, it doesn’t matter which drug you take: The risk reduction of 25% and 0.1% ,though looks different, is in fact the the same for both the drugs
How can Drug A & B have the same effect ??
It is because the percentages are just expressed differently as in absolute vs relative risk. Risk or chances of you getting/preventing/treating a disease is usually expressed in absolute or relative risk. The risk always appear bigger when expressed in relative risk than absolute risk as you can see.
- Drug A (25% reduction) is expressed as Relative Risk
- Drug B (0.1% reduction) is expressed as Absolute risk
For example, consider 1000 women who did not take the drug A/ B and another who did take. Within 10 years, 4 die in the first group an 3 in the second group which did take the drug of cancer as shown in the picture. You can see how we got the absolute and relative risk reduction.
How do people use this information?
- Drug Company: A drug company will always write risk reduction in relative risk to make the benefits look great and the side effects in absolute risks. So you might read an article that Drug A will lower the risk of deaths by 25% and the risks of developing side effects is 1%. That is great drug you will think!
- Caring Doctor: A caring doctor will usually express the risk in absolute terms and avoid percentages. So if the risk is .1% , he will say we will have to treat 1000 people with this drug to prevent one death(shown in the pic). This concept is called Numbers Needed to Treat (NNT).That is not that great as you previously thought!
- Risk/chance can be expressed in relative risk or absolute risk.
- Relative risk is always higher than absolute risks and hence often misread.
- It is better to use absolute risk or the Numbers Needed to Treat (NNT) concept to get the true picture.