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Abstract Background The magnitude of the superiority of antipsychotics over placebo is debated. One reason is that the effect-size index which is usually used in meta-analyses is in standard deviation units. Many other indices, some of which are more intuitive, exist. Methods We explain the formulae, advantages, and limitations of 13 effect-size indices: Mean Difference (MD), Standardized-Mean-Difference (SMD), Correlation Coefficient, Ratio-of-Means (RoM, endpoint and change data), Improvement Fraction (IF), Drug-Response Fraction (DRF), Minimally-Clinically-Important-Difference-Units (MCIDU), Number-Needed-to-Treat-derived from SMD (NNT), Odds Ratio (OR), Relative Risk (RR), and Risk Difference (RD) derived from SMD, Drug-response and Placebo-response in percent. We applied these indices to meta-analyses comparing antipsychotic drugs with placebo for acute schizophrenia. Results The difference of all antipsychotics pooled vs placebo (105 trials with 22741 participants) was: MD 9.4 (95% CI 8.4,10.2) PANSS points, SMD 0.47 (0.42,0.51), Correlation coefficient 0.23 (0.21,0.25), RoM endpoint 0.83 (0.81,0.85), RoM change 1.94 (1.84,2.02), IF (%) 49 (46,51), DRF (%) 94 (84,102), MCIDU 0.63 (0.56,0.68), NNT 5 (5,6), OR 2.34 (2.14, 2.52), RR 1.67 (1.59,1.73), RD 20% (18–22), and 50% (48, 52) improved on drug compared to 30% on placebo. Results of individual drugs compared to placebo are presented, as well. Conclusions Taken together these indices show a substantial, but not a large superiority of antipsychotics compared to placebo. The general chronicity of the patients in the trials must be considered. Future meta-analyses should report other effect size indices in addition to the Standardized-Mean-Difference, in particular percentage responders in the drug and placebo groups. They can be easily derived and would enhance the interpretation of research findings.

Original publication




Journal article


Schizophrenia Bulletin


Oxford University Press (OUP)

Publication Date