How effective is that treatment? The number needed to treat.

The Number Needed to Treat (NNT) is a tool useful for comparing the relative effectiveness of two or more therapeutic or preventive interventions. The NNT may be presented by authors of a clinical research article, or, if not provided, may be calculated by the reader if the authors have reported outcomes as positive or negative per research subject. The NNT is simply calculated as the inverse of the absolute risk reduction. The NNT is most meaningful when reported within a confidence interval and when describing clinical trials of higher validity such as randomized controlled trials and meta-analyses of such trials. Several example NNTs from the dental and medical literature are reported.