Background: Randomized controlled trials (RCTs) are the gold standard for trials assessing the effects of therapeutic interventions and provide the best evidence to inform and guide clinical decision-making. Therefore, it is important to understand how they are conducted and how to evaluate the strength of evidence. Bias is a systematic tendency to produce an outcome that differs from the underlying truth. Bias in clinical trials falls into four categories: selection bias, performance bias, detection bias and attrition bias. There are numerous tools to assess methodological quality of primary studies. Understanding the relationship between the risk of bias and the effect size can help plan research to monitor the presence of such bias.

Objectives: Our work has three objectives. First, provide an analysis of the relationship between bias and effect size. Second, analyze the bias and reporting checklists (example CONSORT). Third, we review and analyze the Cochrane Collaboration’s tool for assessing risk of bias.

Methods: Systematic review.

Results: The methodological quality of studies can have a substantial impact on estimates of treatment effect, which may affect the validity of the conclusions.

Conclusions: Our work helps to identify areas of strength and weakness in the existing evidence and to formulate recommendations to improve the conduct and value of future research.