The number needed to treat, or NNT, is the number of patients who need to be treated to prevent one additional bad outcome, calculated as 1/Absolute Risk Reduction.

So, let's look at our hypothetical diabetes blindness drug example.

Let's say the risk for blindness in a patient with diabetes over a 5-year period is 2 in 100 (2%) in a group of patients treated conventionally and 1 in 100 (1%) in patients treated with a new drug. So the absolute difference is derived by simply subtracting the two risks: 2% - 1% = 1%.

The number needed to treat would be 1 / 1% (or .01) = 100.

So you would need to treat 100 people with diabetes for 5 years in order to prevent one case of blindness. You can see that this is an important way to look at new claims about new drugs.

This is a brief introduction to the concept of NNTs. More information, including different ways to calculate NNTs, is available here.