Data extraction
Two authors used pre-designed tables to extract data independently from each of the eligible studies. Disagreements between the two investigators were resolved by discussion or consensus with a third author. We extracted the characteristics of each study and recorded the following data: first author, year of publication, study design, characteristics of the study population, sample size and details related to the methodological quality and results. The numeric results, statistics used and p values were extracted for each outcome. We attempted to contact the author of the original report to obtain further details when the any of the above information was unclear.