A systematic review of empirical studies on methodology and burden of informal patient payments in health systems.

There are a number of strategies for the measurement of IPPs with different strengths and weaknesses. Most applied strategies for general public were quantitative surveys recruiting more than 1000 participants using a face-to-face structured interview, and then qualitative studies on less than 150 health care providers, with focus group discussion. This review provides a comprehensive picture of current informal patients' payments measurement tools, which helps researchers in future investigations.