A frequently unpublicized, but significant, number to understand when interpreting studies:

Generally, you will see fewer heart attacks in the statin group (about 30% fewer in one real-world trial). Reducing the risk by a third sounds like a lot, which is one reason many hundreds of thousands of men with no sign of heart disease take statins. But that number is meaningless unless you take into account the percentage of men in both groups who have heart attacks in the first place. If those people represent only a tiny fraction of the two populations, an improvement of 30% isn’t much–maybe one heart attack fewer in a group of thousands.

The effort to handicap those odds is where NNT comes in. It answers the question, How many people have to take this drug to avoid one heart attack? The same principle can be applied to avoiding one recurrence of cancer or stroke or whatever end point you choose to measure. In healthy men, the NNT for statins is about 50 (depending on which of dozens of statins is taken, age, family history, lifestyle and so on). So 50 men have to take these drugs in order to prevent a single–not necessarily fatal, heart attack.

(via Schwitzer)