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NNActa Psychiatr Scand 2008: 117: 412–419 All rights reserved DOI: 10.1111/j.1600-0447.2008.01194.x

Copyright Ó 2008 The Author Journal Compilation Ó 2008 Blackwell Munksgaard
ACTA PSYCHIATRICA SCANDINAVICA

Clinical overview

Compelling or irrelevant? Using number needed to treat can help decide
Citrome L. Compelling or irrelevant? Using number needed to treat can help decide.Objective: The metric of number needed to treat (NNT), defined as the number of patients who need to be treated to achieve one additional favorable outcome, can help clinicians appraise claims that one intervention is meaningfully superior to the other. Method: A review of the use of NNT to evaluate the differences between interventions in the treatment of depression, schizophrenia and bipolar disorder.Instead of using disparate measures such as point change on a rating scale, kilograms gained over time or relative differences, results can be converted into a common unit of measure – Ôpatient unitsÕ – so that the clinician can anticipate how often actual differences between interventions would be expected to be observed. Calculation of NNT is demonstrated using reports published in the psychiatricliterature, together with different graphical techniques to display this. Results: Clinical trial results expressed as NNT can be easily summarized and communicated effectively to patients, their families and payers. Limitations include ensuring that the NNT metric is calculated from well-designed and well-conducted research that enrolls subjects similar to patients that one treats in actual clinicalpractice, with doses of medications similar to what is used in the Ôreal worldÕ. Direct calculation of NNT is limited to binary or dichotomous outcomes. Conclusion: Using NNT can help predict treatment response in terms of both efficacy and tolerability.
Clinical recommendations

L. Citrome
Department of Psychiatry, New York University School of Medicine, the Nathan S. Kline Institute forPsychiatric Research, Orangeburg, NY, USA

Key words: clinical trials; evidence-based medicine; number needed to treat Leslie Citrome, MD, MPH, Nathan S. Kline Institute for Psychiatric Research, 140 Old Orangeburg Road, Orangeburg, NY 10962, USA. E-mail: citrome@nki.rfmh.org Accepted for publication March 28, 2008

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NNT can be used to appraise the clinical relevance of a statisticallysignificant result. NNT can be used to assess both the advantages and disadvantages between two competing interventions. Before agreeing that a statistically significant clinical trial result is important to the clinician and patient, the NNT can be calculated to determine if this difference will likely be encountered in day-today clinical practice; NNT will inform if the clinical trial result is compellingor if it is irrelevant.

Additional comments

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NNT is an effect size and is independent of statistical significance or ÔP-valueÕ; thus NNT is best used together with its confidence interval. NNT is best calculated from well-designed and well-conducted research. NNT can only be calculated for binary (yes ⁄ no) outcomes; other measures of effect size will be needed for continuousvariables or alternatively the continuous outcome will need to be converted to a binary one.

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NNT clarifies clinical significance
Introduction
35% Percent of patients in remission at 6 weeks 34% 33% 32% 31% 30% Drug A Drug B

Clinical trials can provide a large amount of data, some of it useful in day-to-day practice, some of it not. Clinicians often struggle to find interventions that make adifference in the wellbeing of patients. It is not always easy to discern whether or not a study result should actually change routine practice. Fortunately, there are now tools available to help gauge clinical significance or clinical relevance, instead of focusing exclusively whether or not a study result is statistically significant at the ÔP < 0.05 levelÕ. These clinician-friendly tools fall...