I love a good puzzle. Tonight my wife told me something amazing: July is the worst month in which to undergo a medical procedure because the risk of a mistake is higher in July than in any other month of the year.

The question is why? What’s so special about July? If you don’t already know the answer, I recommend you pause reading this article now and see if you can come up with the answer.

My first thought was that in July many doctors are on vacation or are mentally distracted by summer vacation plans. That explanation is a bit like the old adage about avoiding a car that was manufactured on a Monday or a Friday (due to back-to-work doldrums and weekend anticipation, respectively). That answer turned out to be wrong. If you’re still pondering, I’ll give you a big hint: it’s related to a certain fixed timetable.

The answer is that July is the month when a brand new crop of interns and residents are unleashed upon the general public. Every July the system is flooded with rookies. And rookies make rookie mistakes. But this sounds like an urban myth, doesn’t it?

I did a bit of google sleuthing (somehow that sounds much cooler than “bing sleuthing”) on this topic and learned about a study which scientifically corroborates this claim. This Time article summarizes a recent study that found a 10% spike in deaths due to medication errors in July, primarily in US counties with teaching hospitals. The study concludes thusly:

“After assessing competing explanations, we concluded that the July mortality spike results at least partly from changes associated with the arrival of new medical residents.”

There’s really never a good time to be in the hospital, but if you believe this study, July is a particularly bad time of year to undergo significant medical treatment at a teaching hospital.