# What is a “number needed to treat?”

*Number needed to treat* is a statistic calculated by epidemiologists (I am an epidemiologist by training) to determine how effective something is.

I was going to put the derivation and the formula here, but I decided that linking to Wikipedia was easier.

The real power of *number needed to treat* is in how easy it is to understand. Let’s say that you do a study with Drug A and do all the statistics and find out that the *number needed to treat* is 3. That means that to avoid getting Disease X, you have to give Drug A to 3 people. Let’s say you do another study with Drug B and find out the number needed to treat is 100 (meaning you have to treat 100 people for the drug to work in 1 of them). Which drug is more effective?

Well, if I want one person to get better, I have to give Drug A to 3 people or Drug B to 100 people.

To put it another way, if I gave Drug A to 100 people, 33 would get better. If I gave Drug B to 100 people, only 1 would get better.

Man, maybe *number needed to treat* isn’t that easy to understand…