In my previous blog entry, I tried to explain why absolute risk reductions was the right number to look at. The trouble is absolute reduction is expressed as a percentage and people generally hate percentages and usually need to do some further mental gymnastics to process the information. So statisticians in the 1980’s came up with another way to look at absolute risk reduction, it’s called the number needed to treat (usually abbreviated NNT). NNT is actually pretty easy to understand. For example, suppose a treatment cures everyone treated, well then the NNT is 1. One person, one treatment, one cure. Similarly, if when you treat two people, one is cured, the NNT is 2. If you need to treat 3 people to cure one, the NNT is 3 and so on.
But amusingly enough, all you need to do to get the NNT is basically flip over the absolute risk reduction percentage. For example, if the absolute risk reduction is 1%, that means 100 people needed to be treated to cure one. This means the NNT is 100 but that is exactly:
1/1%
Of course, since fractional people are kind of weird, NNT is always rounded up to the next integer. So for example if the absolute risk reduction is 6%, the number needed to treat would mathematically be 1/6% or 16.6666, but we say the NNT in this case is 17.
Yes, NNT gets more complicated if a treatment could conceivably harm some people and help others, but in most cases, just flipping over the absolute risk reduction and rounding up gives you the NNT.
While I suggested in my previous blog that pharmaceutical companies should be required to give absolute risk reduction whenever they give relative risk reduction in an advertisement, requiring them to add the NNT in the same font etc. might even be a better idea!