Medical errors tend to fall into two buckets. There are the mistakes that happen when doctors set a wrong plan: when they prescribe the wrong medication, for example, thinking it was the right treatment. Then there are the errors that occur when doctors set the right plan but don't follow it — when messy handwriting means a patient gets the wrong drug dosage, for example, or when a surgeon operates on the wrong body part (yes, this actually happens).
"Something like 2 to 3 percent of people who go into the hospital are going to have some pretty severe harm as a result," says Don Berwick, the Obama administration's former Medicare administrator. "Australian studies show that the rate might be as high as 12 percent. The harder you look, and the more you study the issue, the more errors you find."