Say you were worried about having a heart attack. Which drug would you rather take:
- Magic Drug A: reduces your risk of a heart attack by 50%
- Magic Drug B: reduces your risk of a heart attack by 1%
Presumably, you'd pick Magic Drug A.
But, what if you found out Magic Drug A and Magic Drug B were really the same drug?
Huh? How can that be? It has to do with something Graham Walker, an emergency department physician at Stanford, calls “statistical trickery.”Here’s how:
- There’s “absolute risk,” your risk of something bad happening, compared to everyone else. It’s a general population risk.
- There’s “relative risk.” This is your reduced risk of something bad happening in relation to something else, such as taking a drug. In our example, it’s our Magic Drug, to reduce risk of that heart attack. For people not working in numbers every day, this takes a moment to get your mind around, so bear with us for a walk-through.Let’s give our Magic Drug to 100 people. Time goes by, and the people who got the drug had 50% fewer heart attacks than 100 people who didn’t get the drug.Here’s where the statistical trickery comes in. How many people who were taking the drug actually had heart attacks? One.
And of the 100 people who did NOT get the drug? How many of them had heart attacks? Two.
So, yes, one heart attack is half as many heart attacks as two. So relatively speaking, it’s a 50% reduction. But, in absolute terms, the risk was reduced from 2% to 1%, a much less compelling 1% reduction.
Diana Stilwell of the Foundation for Informed Decision Making compares it to shopping. “It’s like going into a store that’s selling things at 20% off. The 20% is only meaningful to you if you know what you’re going to be buying.”
If you’re buying a pair of shoes, your 20% savings could be a lot of money. If you’re buying a loaf of bread, not so much.
Walker is one of the founders of a terrific website, theNNT.com. Visitors can get easily digestible information about risks and benefits for a range of drugs and other medical treatments. Walker says the website is entirely supported by the handful of doctors who put it together. For fun, you can watch Walker’s video about the NNT.