Which type of review is subject to bias associated with personal opinion?

A narrative review is characterized by its qualitative approach, often summarizing existing literature without a rigorous methodology to evaluate the studies included. This format allows for the inclusion of the author's interpretations and opinions, which may lead to bias. The reviewer may selectively include studies that support their viewpoint, while overlooking or downplaying contrary evidence. As a result, the conclusions drawn in narrative reviews can be heavily influenced by personal biases and subjective judgments rather than an objective analysis of the data available.

In contrast, systematic reviews and meta-analyses are structured methodologies that aim to minimize bias through specific criteria for study selection and rigorous data analysis. Systematic reviews assess and synthesize evidence from multiple studies systematically and transparently, while meta-analyses quantitatively combine results from different studies to provide a clearer picture of the overall evidence. Research studies, while they can have their own biases, are typically designed to test specific hypotheses and present original findings, which differ from the interpretive nature of narrative reviews.