To err is human, and deadly when those humans are doctors. Major surgical errors are pretty rare—some estimates put them at one in 12,000 surgeries—but they do happen, even egregious ones like leaving instruments behind or performing the wrong surgery altogether, and Mayo Clinic researchers recently set out to find out why by analyzing 69 such events that occurred over the course of 1.5 million invasive surgeries in a five-year period.

Reporting in the journal Surgery, they found that "never events" (meaning ones that should never happen) occurred once in every 22,000 surgeries in this window, and that these include 24 wrong surgeries, 22 involving the wrong side or site, 18 instances of leaving an object in the patient, and five wrong implants.

The researchers used the Human Factors Analysis and Classification System—initially developed to investigate military aviation accidents—and organized errors into four types, reports the Mayo Clinic: preconditions for action, like overconfidence and inadequate communication; unsafe actions, like breaking rules or confirmation bias; oversight factors, like poor staffing and planning; and organizational influences, such as problems with operational processes.

They found that most major errors involve an average of nine missteps, and that mental states of doctors and nurses (i.e. overconfidence and stress) were the most common source of errors, reports Pacific Standard.

And while they didn't get specific on how to mitigate these errors, they did note that lightening the workload should ease the error rate. (Hospitals appear to profit from these mistakes.)

This article originally appeared on Newser: Why Do Docs Botch Surgeries?

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