Calculating Risks and Number-Needed-to-Treat: A Method of Data Interpretation

OBJECTIVE: To define four measures of association relative risk (RR), absolute risk reduction (ARR), relative risk reduction (RRR), and number-needed-to-treat (NNT), demonstrate the method of calculating these values, and discuss the clinical relevance of each value. DATA SOURCES: MEDLINE 1991 to present and bibliographies of pertinent articles. STUDY SELECTION: Articles designed to assess statistical data interpretation and/or discuss measures of association were evaluated. DATA SYNTHESIS: The RR, ARR, RRR, and NNT were calculated for the study endpoints. For the primary endpoint (nonfatal MI or death from CHD), RR=0.70; 2.26% (ARR) of the patients receiving pravastatin were spared the event; 30% (RRR) of the baseline risk was removed as a result of pravastatin therapy and 44 patients need to be treated to prevent an adverse event. CONCLUSION: RR, ARR, RRR, and NNT can assist the reader in determining the association between treatment and a health outcome and can be calculated from results of studies reporting dichotomous endpoints. These values can assist in the interpretation of clinical trials to determine the magnitude of benefit from therapy.