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How to Obtain NNT from Cohen's d: Comparison of Two Methods

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  • Toshi A Furukawa
  • Stefan Leucht

Abstract

Background: In the literature we find many indices of size of treatment effect (effect size: ES). The preferred index of treatment effect in evidence-based medicine is the number needed to treat (NNT), while the most common one in the medical literature is Cohen's d when the outcome is continuous. There is confusion about how to convert Cohen's d into NNT. Methods: We conducted meta-analyses of individual patient data from 10 randomized controlled trials of second generation antipsychotics for schizophrenia (n = 4278) to produce Cohen's d and NNTs for various definitions of response, using cutoffs of 10% through 90% reduction on the symptom severity scale. These actual NNTs were compared with NNTs calculated from Cohen's d according to two proposed methods in the literature (Kraemer, et al., Biological Psychiatry, 2006; Furukawa, Lancet, 1999). Results: NNTs from Kraemer's method overlapped with the actual NNTs in 56%, while those based on Furukawa's method fell within the observed ranges of NNTs in 97% of the examined instances. For various definitions of response corresponding with 10% through 70% symptom reduction where we observed a non-small number of responders, the degree of agreement for the former method was at a chance level (ANOVA ICC of 0.12, p = 0.22) but that for the latter method was ANOVA ICC of 0.86 (95%CI: 0.55 to 0.95, p

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  • Toshi A Furukawa & Stefan Leucht, 2011. "How to Obtain NNT from Cohen's d: Comparison of Two Methods," PLOS ONE, Public Library of Science, vol. 6(4), pages 1-5, April.
    • Handle: RePEc:plo:pone00:0019070
    DOI: 10.1371/journal.pone.0019070
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    Cited by:

    1. Jørund Gåsemyr & Bent Natvig & Ingunn Fride Tvete, 2016. "Estimating Response Ratios from Continuous Outcome Data," Methodology and Computing in Applied Probability, Springer, vol. 18(1), pages 137-151, March.
    2. Valerio Pellegrini & Valeria De Cristofaro & Marco Salvati & Mauro Giacomantonio & Luigi Leone, 2021. "Social Exclusion and Anti-Immigration Attitudes in Europe: The mediating role of Interpersonal Trust," Social Indicators Research: An International and Interdisciplinary Journal for Quality-of-Life Measurement, Springer, vol. 155(2), pages 697-724, June.

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