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If you notice a major journal(s) for your topic are not indexed in your chosen databases then do a search for the journal title using the Journals List then search each issue, page-by-page, to find additional articles to include in your review.
Backward Searching: You can use an article's reference list to find additional articles to search - this is often referred to as backwards searching. Copy and paste the article's citation into Google Scholar to find it.
Forward Searching: You can also see the impact of a specific article by looking at who cited it - this is referred to as forward searching. Copy and paste the citation into Google Scholar then click the "Cited By" button beneath the abstract on the results page to see articles that have incorporated that research.