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Not all journal articles are peer reviewed because not all journals are peer reviewed.
Articles in peer-reviewed journals are reviewed by a group of the writer's peers (other expert academics in their field) before the articles are published.
You can find out if a journal (and its articles) is peer reviewed by searching for the journal title in Ulrich's.
Is Your Journal Article Peer Reviewed?
How do you know if an article is from a peer-reviewed journal? Some databases allow you to limit your search to peer-reviewed journals. For other databases, you need to look up the title of the journal in Ulrich's Directory.