Quality of evidence

Two authors (NAM, ES) assessed the overall quality of the evidence of the studies using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) tool.³⁸ As all included studies were non-randomised controlled studies or quasi-randomised controlled studies, they were automatically assigned a baseline rating of low quality. Studies had potential to be upgraded if there was a large effect size, evidence of a dose–response relationship, or when all possible confounders would have reduced the observed effect. Similarly, studies could be downgraded if there were serious concerns relating to risk of bias, inconsistency of results, indirectness of outcome measure, imprecision of the effect estimate or publication bias.³⁸ The quality of the studies was judged as high (further research is very unlikely to change the confidence in the effect estimates), moderate (further research is likely to have an important impact on the confidence in the effect and may change the estimate), low (further research is very likely to have an important impact on the confidence in the effect and is likely to change the estimate) and very low (any estimate of the effect is very uncertain).³⁸