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We agree with Zheng and colleagues that a statistical measure of heterogeneity is an important consideration in selecting a fixed or random effects model. However, it should not be considered the sole criterion. In meta-analyses that include small numbers of trials, such as ours, results of tests for heterogeneity should be interpreted with caution, as they may be underpowered. A fixed effects model has been advocated when meta-analysing small number of studies (less than five trials), given the concern about inaccurate estimation of