Journals - both academic and professional are important sources of up-to-date info.
Articles in academic journal have been peer-reviewed (that the articles are subject to the srcutiny of experts before publication.
Meta analysis - a type of data analysis in which the results of several studies are combined and analyzed as if they wre the resulst of one large study.
Review articles - summarize previous research finding & inform you of the state of existing research.
Research articles - are reports of empirical research describing one or a few related studies.
Theses - exhausted review and includes several empircal chapters.
Conference proceedings - useful for latest research, very up-to-date and information source is considered very valuable.
Searching for literature - MODERN techno helps save time, the are comprehensive in their listing and reviews of references and gaing access to them relatively inexpensive and you can focus on material more central to the project.