| Summary: | In observational studies, groups of interest may be carved out of predictors of interest. Thus, for example, if cardiovascular (CVS) health at age 50 years is the predictor of interest for dementia as the long-term outcome, groups of interest could comprise persons with poor, intermediate, and optimal CVS health at age 50. These groups would almost certainly be unbalanced in terms of sample size and duration of follow-up when incident dementia is assessed. The present article is a companion to the previous article in this column; it explains why the duration of follow-up needs to be factored into the sample size in each group, making person-years rather than persons as the unit for risk assessment. Next, this article explains how to reverse calculate an estimate of the number needed to treat (NNT) statistic in such situations. Finally, this article explains why the NNT so estimated is an approximation and not a true estimate of the NNT in the population.
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