Dissertations and theses are long works produced as the written culmination of many Masters and PhD programs. They are excellent resources in other contexts, but you may want to exclude them if you are performing a review of peer-reviewed journals. Here's how to avoid seeing them in your search results:
ProQuest offers the advanced search option of limiting by document type. Either select Journal Article as your document type,
Or, deselect the dissertations/theses fields:
Also note that dissertations and theses are typically not published by a scholarly journal, but are instead published or made available by a university. It is always important to pay attention to the publisher's information on any article you wish to use.