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Abstract: The “number-needed-to-treat” (NNT) was introduced about 15 years ago and has gained widespread use. It has been claimed to be “easy to understand” and gives “intuitive meaning”. When used to measure the effectiveness of interventions targeting chronic disease processes e.g. atherosclerosis and osteoporosis, NNT (as well as relative and absolute risk reduction) does not capture the crucial time component, a fact that has important consequences: NNT varies over time, it may not mean that adverse events (fractures, myocardial infarctions etc.) are avoided, but simply that they are postponed. Finally, empirical studies indicate that lay people and doctors misunderstand NNT. We recommend that NNT be used with considerable care. There is probably no single effect measure that is able to convey all necessary information.