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Ratio of means for analyzing continuous outcomes in meta-analysis performed as well as mean difference methods
Authors:Friedrich Jan O  Adhikari Neill K J  Beyene Joseph
Affiliation:a Department of Medicine, University of Toronto, Toronto, Ontario, Canada
b Interdepartmental Division of Critical Care, University of Toronto, Toronto, Ontario, Canada
c Department of Medicine, St. Michael’s Hospital, Toronto, Ontario, Canada M5B 1W8
d Department of Critical Care, St. Michael’s Hospital, Toronto, Ontario, Canada M5B 1W8
e The Keenan Research Centre in the Li Ka Shing Knowledge Institute, St. Michael’s Hospital, Toronto, Ontario, Canada M5B 1W8
f Department of Critical Care Medicine and Sunnybrook Research Institute, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada M4N 3M5
g Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada
h Child Health Evaluative Sciences, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada M5G 1E2
Abstract:

Objective

Meta-analyses of continuous outcomes typically use mean differences (MDs) or standardized mean differences (SMDs) (MD in pooled standard deviation units). Ratio of means (RoM) is an alternative effect measure that performs comparably in simulation. We compared treatment effects and heterogeneity for RoM, MD, and SMD using empiric data.

Study Design and Setting

From the Cochrane Database (2008, issue 1), we included systematic reviews reporting continuous outcomes, selected the meta-analysis with the most (and ≥five) trials, and calculated MD (where possible), SMD, and RoM. For each pair of effect measures, we compared P-values separately for treatment effect and heterogeneity and assessed asymmetry of discordant pairs (statistically significant result for only one of two measures).

Results

Two hundred thirty-two of 5,053 reviews were included. Measures demonstrated similar treatment effects, with ≤6% discordant pairs and no asymmetry. A 0.5 SMD increase corresponded to 22 (95% confidence interval: 19, 24)% increase using RoM. There was less heterogeneity in RoM vs. MD (n = 143, P = 0.007), SMD vs. RoM (n = 232, P = 0.005), and SMD vs. MD (n = 143, P = 0.004). Comparing discordant pairs, fewer meta-analyses showed significant heterogeneity with SMD vs. RoM (P = 0.04), consistent with the known bias of SMD.

Conclusion

Empiric data from diverse meta-analyses demonstrate similar treatment effects and no large differences in heterogeneity of RoM compared with difference-based methods.
Keywords:Meta-analysis   Mean differences   Ratio of means   Statistical methods   Method comparisons   Standardized mean difference
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