The initial discovery phases of pharmacogenetic studies typically emphasize statistical significance and replication. These yardsticks are necessary for establishing the scientific reliability of a finding, but tell us nothing about how valuable the information is for clinical decision making. Here, the well established concept of ‘Number Needed to Screen’ is valuable, since it incorporates both the frequency of a marker and the magnitude of its effect [Rembold 1998]. The NNS captures how many patients need to receive a test for every patient whose outcome is altered. Smaller NNS values are generally better, but there is no single threshold. If the goal is to avoid a severe adverse event, larger NNS might be reasonable, while quantitative improvements in response might require smaller NNS values to make sense clinically.
