Number Needed to Treat: A Statistic Relevant for Physical Therapists

Abstract

The number needed to treat (NNT) is a method of reporting outcomes from clinical trials.1 Treatment efficacy is determined by evaluating the outcome of one treatment relative to another treatment or to a control group when the only difference between the groups is the intervention of interest. The NNT can also be used to express the size of the outcome of one treatment relative to another. The NNT is expressed in terms designed to help decide whether the intervention might be valuable in clinical practice: the number of patients who need to be treated before a therapist can be sure that one patient improved who would not have improved without the intervention. For example, when comparing treatment X and treatment Y, an NNT score of 5 for treatment X indicates that, on average, after treating 5 patients, treatment X will have achieved one more positive outcome than if treatment Y had been used. The NNT does not tell the clinician which of those 5 patients will respond, only that 1 patient is likely to do so. The NNT was described in 1988 by Laupacis et al,2 and, although its use is becoming more popular, it is still not widely used. A search of MEDLINE back to 1991 using the search terms “number needed to treat” or “NNT” identified 121 citations reporting NNT information. Of these citations, only 3 concerned outcomes of physical therapy3–5 and, of these, only one5 described the use of NNT in a journal with content relevant to physical therapists. The NNT can be used to gauge the relative effectiveness of different treatments in restoring normal function or preventing future disability. If the treatment has a potentially harmful outcome (eg, stroke, headache, death from cervical manipulation), a similar calculation can be used …