What type of bias occurs when non-published studies are excluded from systematic reviews?

Publication bias occurs when the outcomes of research studies influence the decision to publish them. In systematic reviews, if non-published studies—typically those reporting negative or inconclusive findings—are excluded, it can create a skewed understanding of the evidence on a given topic. This bias can result in an overestimation of effects because the literature may mainly reflect studies with positive results, leading researchers and practitioners to draw conclusions based on incomplete information. The inclusion of only published studies fails to accurately represent the entirety of available evidence, thus compromising the review's validity.

In contrast, language bias relates to the likelihood of studies being published based on the language of the publication, while selection bias involves the way participants or studies are selected for inclusion in a review. Funding bias pertains to the influence of the funding source on the results or interpretation of the research. Understanding and addressing publication bias is essential for ensuring that systematic reviews and meta-analyses present a more accurate and comprehensive view of the evidence.