3 Kinds of Literature Review
1.RESEARCH LITERATURE- these are empirically-based, like scientific paper, theses, and dissertations, both published and unpublished, coming from local and foreign sources.
2. CONCEPTUAL LITERATURE- it contains foreign and/or local literature coming from books relevant to the study, but are data-free or non-empirical material.
3. GRAY LITERATURE- from other sources such as magazines, newspapers, flyers, and other forms of material.