"Expert opinion has an important role to play in evidence-based healthcare, as it can be used to either complement empirical evidence or, in the absence of scientific evidence, stand alone as the best available evidence" (McArthur et al, 2015).
Expert opinion and policy analysis systematic reviews focus on the synthesis of narrative text and/or policy..."to review and synthesize current expert opinion, text or policy on a certain phenomena" (Munn, 2018).
Munn, Z., Stern, C., Aromataris, E., Lockwood, C., & Jordan, Z. (2018). What kind of systematic review should I conduct? A proposed typology and guidance for systematic reviewers in the medical and health sciences. BMC medical research methodology, 18(1), 1-9. Full Text
McArthur, A., Klugárová, J., Yan, H., & Florescu, S. (2015). Innovations in the systematic review of text and opinion. JBI Evidence Implementation, 13(3), 188-195. Full Text
Jordan, Z., Konno, R., & Mu, P. (2011). Synthesizing evidence from narrative, text and opinion. Lippincott Williams & Wilkins. Full Text PDF
References
Munn, Z., Stern, C., Aromataris, E., Lockwood, C., & Jordan, Z. (2018). What kind of systematic review should I conduct? A proposed typology and guidance for systematic reviewers in the medical and health sciences. BMC medical research methodology, 18(1), 1-9. Full Text
McArthur, A., Klugárová, J., Yan, H., & Florescu, S. (2015). Innovations in the systematic review of text and opinion. JBI Evidence Implementation, 13(3), 188-195. Full Text