PubMed comprises more than 39 million citations for biomedical literature from MEDLINE, life science journals, and online books.
Fact Sheet: How are Medline, Pubmed and Pubmed Central different?
Many researchers do thorough studies in order to complete a dissertation or thesis, but these studies may not always be published later as journal articles. It is important to search for dissertations/theses on your topic, especially as these studies can help you as you are designing your research study.