Have you ever heard of the term Number Needed to Harm? The surgeon who wired my broken elbow back together used this phrase. It was how she decided which method to use when surgically repairing my elbow. I’m 12-years old and very interested in numbers and statistics. I’d like to understand this idea.

The statistical method called Number Needed to Harm (NNH) is a way to evaluate data pooled from many studies. It is used when comparing two different treatment techniques for the same problem.

Number Needed to Harm refers to the number of patients treated in a certain way before someone gets hurt or has a complication. It might seem like an odd way to look at something. But using the Number Needed to Harm is more meaningful to a surgeon than using mathematical ratios or odds.

Using the Number Needed to Harm gives the surgeon a better perspective of the benefits and risks of a procedure. Likewise, it gives patients a way to make an informed choice about surgery. Your surgeon may have read a recent study on two different pinning methods for elbow fractures. The results were reported using Number Needed to Harm.

Without going into too much detail, they compared using a crossed pin to straight (lateral) pinning method for elbow fractures that had been displaced. A displaced fracture means the broken bone fragment separates and shifts away from the main bone. The crossed fixation technique may increase the risk of injuring the ulnar nerve, which runs through the affected area.

The results showed that for every 28 patients who were treated with the crossed pin fixation method, one would have an ulnar nerve injury. This was the number of patients who would have to be treated by crossed pinning to have one more nerve injury compared with the lateral pin method.

Using the Number Needed to Harm (NNH) as the only measure to show that one method is safer than another is not necessarily recommended. Together with other measuring methods, it provides some additional information for patients and surgeons. This information can be used when deciding on the relative risks and benefits of this particular fixation technique.