European Journal of Epidemiology | 2019
Umbrella reviews: what they are and why we need them
Abstract
Evidence in clinical research and public health is hierarchical. From animal studies to observational epidemiology, randomized control trials and their synthesis, there is a pipeline of different study designs providing different nature and quality of information. Systematic reviews and meta-analyses stand on top of this hierarchy most of the times and are key components of evidence-based medicine [1]. Their production and publication has increased exponentially in the last decade across almost all disciplines with approximately 20,000 records labeled as meta-analysis in 2018 compared to 3300 in 2008. One would expect that this growth would reflect the increase in the number of primary observational and experimental studies but this is not true because all Pubmed-indexed items increased by 153% from 1991 to 2014 [2]. This abundance of published evidence synthesis cannot be considered only positive. It is not uncommon for metaanalyses on the same research question to reach different conclusions even when published within the same year. This unavoidably leads to confusion and debate in clinicians and public health policy makers on where to base their decisions on. There are several preventive measures proposed in order to minimize bias in meta-analyses. A priori publication of protocols is highly encouraged and each analytical step and every subjective judgment call should be reported [3]. Finally there are tools to appraise the reporting [4] and quality [5] of the published systematic reviews and meta-analyses. Knowledge gap