The authors regret, in the above-mentioned article, we discovered an error related to standards for adequate precision in systematic reviews of continuous variables. The relevant paragraph reads as follows: “Because it may give false reassurance, we hesitate to offer a rule-of-thumb threshold for the absolute number of patients required for adequate precision for continuous variables. For example, using the usual standards of α (0.05) and β (0.20), and an effect size of 0.2 standard deviations, representing a small effect, requires a total samples size of approximately 400 (200 per group) - a sample size that may not be sufficient to ensure prognostic balance.” The sample size stated is incorrect: the correct number is 800 (400 per group). The authors would like to apologise for any inconvenience caused. The authors are grateful to Mark Chatfield for pointing out this error.