Studies that are included in a systematic review may include biases in their results or conclusions. Bias can lead to either underestimation or overestimation of the true effect of an intervention, with varying degrees of impact. Biases may arise from the actions of primary study investigators, review authors, or limitations in the research process, and can be influenced by conflicts of interest (Boutron et al., 2023). Studies should be evaluated for risk of bias with a tool that fits the study designs included in your systematic review.
Per the Cochrane Handbook:
"Methodological quality refers to critical appraisal of a study or systematic review and the extent to which study authors conducted and reported their research to the highest possible standard. Bias refers to systematic deviation of results or inferences from the truth. These deviations can occur as a result of flaws in design, conduct, analysis, and/or reporting. It is not always possible to know whether an estimate is biased even if there is a flaw in the study; further, it is difficult to quantify and at times to predict the direction of bias" (Higgins et al., 2023).
The most recent version of the Cochrane handbook also states that "Most recent tools for assessing the internal validity of findings from quantitative studies in health now focus on risk of bias, whereas previous tools targeted the broader notion of ‘methodological quality’" (Boutron et al., 2023).
Types of bias can also include:
For more information on bias see:
Cochrane Handbook Chapter 7: Considering Bias and Conflicts of Interest Among the Included Studies.
Cochrane Handbook Chapter 8: Assessing Risk of Bias in a Randomized Trial.
Finding What Works in Health Care: Reporting Bias (IOM, 2011)
References