Net Reclassification Index Statistics Do Not Help Assess New Risk Models.

When evaluating a new risk factor for disease (eg, a measurement from imaging studies), many investigators examine its value above and beyond existing biomarkers and risk factors. They compare the performance of an "old" risk model using established predictors and a "new" risk model that adds the new factor. Net reclassification index (NRI) statistics are a family of metrics for comparing two risk models. NRI statistics became popular in some medical fields and have appeared in high-impact journals. This article reviews NRI statistics and describes several issues with them. Problems include unacceptable statistical behavior, incorrect statistical inferences, and lack of interpretability. NRI statistics are unhelpful (at best) and misleading (at worst).