[Risk of bias assessment: (1) overview].

This paper reviews the concept of risk of bias, followed by demonstrating why assessment of risk of bias in systematic reviews should be different from that of quality of evidence, methodological quality, reporting quality, precision, and external validity. We also discuss the recent development of tools for risk of bias assessment, the problems with the tools themselves, and the challenges in using these tools. This review may help systematic reviewers understand risk of bias assessment and the use of assessment tools.