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 academics in their field) before the articles are published.
It might sound confusing, but you know that an article is peer-reviewed if it comes from a peer-reviewed journal.
You can find out if a journal (and its articles) is peer-reviewed by searching for the journal title in Ulrich's.
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