It doesn't matter. If you can calculate pooled estimate effect from your included studies, then nothing to worry about. You can perform sub-group analysis using different types of study.

Hi Behzad,

You refer to assessing "quality" and "checklists" for the different study designs. I am not sure if you are alluding to risk of bias assessment (which typically uses checklists such as RoB2, ROBINS-I, NOS, among others) or the GRADE assessment of quality of evidence (of which risk of bias assessment is only one of the criteria for rating down the quality of evidence). Since you explicitly mentioned checklists, I shall assume it is the latter.

As you pointed out, there are different checklists for assessing risk of bias for different study designs, i.e. there are different criteria used for each study design. It therefore makes sense and is common practice to summarize risk of bias assessments according to study design. The advantage of doing so is particularly apparent when you are summarizing risk of bias using a summary figure (for which the checklist criteria comprise the column headings): https://handbook-5-1.cochrane.org/chapter_8/figure_8_6_c_example_of_a_risk_of_bias_summary_figure.htm

Best Regards,

Elenore

Reporting them separately is a good idea as the checklists for different designs are different. However, you might be able to determine the proportion of studies that are "high quality" across designs if the checklists you use enable that distinction.

I agree with Anees - I think you will also see differences in Strength of Evidence related to study design, so in separating you will be able to look for the strength of evidence in each category.