Clinical research VI. The clinical relevance

Abstract

Usually, in clinical practice the maneuver selected is the one that achieve a favorable outcome with a direct percentage of superiority of at least 10 %, or when the number needed to treat ≈ 10. While this percentage difference is practical to estimate the magnitud of an association we need to differentiate the impact measures (attributable risk, preventable fraction), measures of association (RR, OR, HR) and frequency measures (incidence and prevalence), applicable when the outcome is nominal. And we must identify ways to measure the strength of association and the magnitude of the association when the outcome variable is quantitative. It is not uncommon to interpret the measures of association as if they were impact measures. For example, for a RR of 0.68, its common to assume a 32 % reduction of the outcome, but, we must consider that this is a relative reduction, which comes from relation of 0.4/0.6, 0.04/0.06, or 0.00004/0.00006, however the direct reduction is 20 % (60 % - 40 %), 2 %, and 2 per 100 000 respectively. Therefore, to estimate the impact of a maneuver it is important to have the direct difference and / or NNT.