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Some databases allow you to narrow your search by specifying just one type of source to find, e.g. books or journal articles or sound recordings. Another option in some databases is to limit your search to find only peer reviewed articles, i.e. articles in academic journals which are reviewed by other scholars and researchers in the field before they are published to check for accuracy and importance of contents. 

For example, the SearchPlus advanced searching page in Readers' Guide allows you to open a pull-down menu of a long list of specific publication types.