Figure 1.

Flow-chart illustrating the article search process. First, we obtained 166 records identified through database searching. No additional records identified through other sources. Second, 89 records remained after duplicates were removed. Third, 58 studies were excluded after records screening. Then the remainder 31 studies were full-text articles assessed for eligibility and 6 studies were excluded. Finally 25 studies were included in the quantitative synthesis (meta-analysis).