I found many articles focus on the topic that I am interested in, but they are "published" on SSRN, not in traditional academic journals. Should I include them in my literature review?
My humble suggestion is to include the working papers and forthcoming papers from SSRN if those papers are related to your area. SSRN is a reliable research network that is devoted to the rapid worldwide dissemination of social science research.
In my opinion, you should include any and all published and unpublished articles in your initial review. You should then assess the quality of the study and potentially exclude some studies. This may include the SSRN papers, but there is no strong reason to believe that published studies are by definition higher quality than unpublished studies. Besides, these SSRN papers may be currently under review and published eventually.
Thus, yes include them and anything else you find.
My humble suggestion is to include the working papers and forthcoming papers from SSRN if those papers are related to your area. SSRN is a reliable research network that is devoted to the rapid worldwide dissemination of social science research.
You should include. But first try to investigate why they were not included in traditional journals and also here might be a valid reason for that. If there is none then go ahe and good luck