Article type
Year
Abstract
Background: Many indices of size of treatment effect (effect size: ES) are found in the literature. The preferred one in evidence-based medicine is the number needed to treat (NNT) while the most common is Cohenás d, when the outcome is continuous. There is confusion about how to convert Cohenás d into NNT.
Methods: We conducted meta-analyses of individual patient data from 10 randomized controlled trials of second generation antipsychotics for schizophrenia ($ox{n}=4278$) to produce Cohenás 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 Cohenás d according to Kraemerás method (Biological Psychiatry, 2006; 59:990-996) and Furukawaás method (The Lancet, 1999; 353:1680).
Results: NNTs from Kraemerás method overlapped with the actual NNTs in 56%, while those based on Furukawaás 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<0.01).
Conclusions: Furukawaás method allows more accurate prediction of NNTs from Cohenás d. Kraemerás method gives a wrong impression that NNT is constant for a given d even when the event rate differs.
Methods: We conducted meta-analyses of individual patient data from 10 randomized controlled trials of second generation antipsychotics for schizophrenia ($ox{n}=4278$) to produce Cohenás 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 Cohenás d according to Kraemerás method (Biological Psychiatry, 2006; 59:990-996) and Furukawaás method (The Lancet, 1999; 353:1680).
Results: NNTs from Kraemerás method overlapped with the actual NNTs in 56%, while those based on Furukawaás 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<0.01).
Conclusions: Furukawaás method allows more accurate prediction of NNTs from Cohenás d. Kraemerás method gives a wrong impression that NNT is constant for a given d even when the event rate differs.