Many systematic reviews include data from non-randomized studies. Are the methods used in the meta-analyses appropriate?

Faber et al. (1) searched MEDLINE for meta-analyses that had been published during 2013 and that included non-randomized studies. Two reviewers then assessed the characteristics and key methodological components in these publications.

Of the initially selected 188 papers, 119 included both randomized and non-randomized intervention studies, and 69 only non-randomized intervention studies. Assessments of bias was reported in 135 papers (72%), but this evaluation referred to confounding bias only in 33 papers (18%). In 130 papers (69%) the design of the non-randomized intervention study was not clearly specified, and it was unclear in 131 papers (70%) if crude or adjusted estimates were used.

The authors conclude that some important methodological aspects of the systematic review process are not adequately reported in meta-analyses that include non-randomized intervention studies.
Reference

  1. Faber T, Ravaud P, Riveros C, Perrodeau C, Dechartres A. Meta-analyses including non-randomized studies of therapeutic interventions: a methodological review. BMC Medical Research Methodology 2016:35 DOI: 10.1186/s12874-016-0136-0