The data abstraction was conducted independently by two investigators (YM, KT). The selected studies were reviewed by using pretested and standardized abstraction format and the following data were extracted: title, authors, year of publication, study site/base (community-based or institution-based), sample size, response rates, and measure of rate with its confidence interval (CI). When there was a discrepancy in data abstraction between the investigators, it was resolved through discussion and consensus.
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