Contribution of observational studies to meta-analyses

Systematic reviews and meta-analyses are crucial tools in evidence-based medicine, providing comprehensive assessments of research findings. Because of their more limited robustness, the inclusion of observational data in meta-analyses raises some concerns and methodological challenges. This study aimed to evaluate the methodological characteristics of systematic reviews and meta-analyses that incorporate both observational studies and randomized controlled trials across diverse medical disciplines.

1-In a first work, we extracted the general and methodological characteristics of these systematic reviews and meta-analyses in duplicate. These data corresponded to the type of intervention studied, the nature of the outcomes of interest, the availability of a protocol, the presence of a justification by the authors for the inclusion of observational studies, the eligibility criteria for the inclusion of studies in systematic reviews/meta-analyses, the tools used to assess the risk of bias for the different study designs, the type of effect estimates (crude or adjusted) and the authors' decision regarding the synthesis of observational studies with randomized controlled trials.

2-In a second work, we considered the first meta-analysis presented in each article combining the two types of design within the same meta-analysis, and studied the statistical methods of synthesis used in these meta-analyses.