Measures of treatment efficacy.

Measures of treatment efficacy are those numbers we think about when we decide whether one treatment is "better" than another. Such measures quantify the differences between treatments and help patients and clinicians make informed choices. The usual measure of treatment efficacy in periodontal research has been the mean difference between treatments in probing level measures. This measure has frustrated clinicians and researchers alike for its failure to communicate the size of the association between treatment and clinical outcome. How does one interpret the clinical relevance of a small mean difference between treatments, such as 0.4 mm? This report compares the advantages and disadvantages of the different measures of treatment efficacy: the mean difference, the relative risk, significance levels (P-values), the risk difference, and, the preventable fraction.