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Peer review is generally a required step for original research to go through before being published in a scholarly journal. A formal peer review process typically involves scientific and editorial review from other researchers with relevant expertise.
Manuscript authors often then take these comments and rework their paper to address the issues identified. Peer review is intended to ensure high quality and careful considerations for rigorous research.
Because peer review is not a requirement for the public posting of preprints, repositories, servers, and indices post clear warnings about this distinction on their platforms. On PubMed CentralĀ® (PMC), this distinction is made clear on each preprint by a large banner at the top of the article denoting that peer review has not occurred yet. Other preprint servers, such as medRxiv, also denote this on their home page and each article record.
(Image Source: https://pmc.ncbi.nlm.nih.gov/about/nihpreprints/)