I want to know if using different studies with different study designs could affect the quality of systematic review? If yes, what things to consider while analysing the data?
Disclaimer: I am not an expert in this field but I believe so. However, you will have to take into consideration how different these studies are, what measure of association is used in the studies, the specific outcome studies( in case you are looking at multiple outcomes). In most cases, you might have to do a separate meta-analysis for the corresponding design, the measure of association and outcome.
This paper suggests that Inclusive Approach - all study designs including randomized control trials (RCTs), quasi-experimental and observational studies) - is preferable.
Tufanaru, Catalin MD, MPH, MClinSci (EBHC); Munn, Zachary PhD; Stephenson, Matthew PhD; Aromataris, Edoardo PhD Fixed or random effects meta-analysis? Common methodological issues in systematic reviews of effectiveness, International Journal of Evidence-Based Healthcare: September 2015 - Volume 13 - Issue 3 - p 196-207