in my opinion, i largely depends on the assumptions related to similarity of the disease (mechanisms) and similarity of the given intervention, eg like a drug dose versus exposure, or eg maturational differences in disease (like eg GBS sepsis in early neonatal life, versus meningitis, more common in 'older neonates and infants. Is this reply useful ?

It depends on your research question and whether or not these studies meet your study eligibility criteria.

thank you karel and Gordon. my study is about PDA closure therapies.do you suggest that the RCT population neonates, newborns, and infants can be pooled together for a meta-analysis?

Gordon L Warren Karel Allegaert

I know nothing about your research topic. You should however format your research question in PICO format if at all possible.

Patient (P):

Intervention (I):

Comparison (C):

Outcome (O):

You and/or other experts in the field should be deciding what constitutes the four components. I hope you are receiving quality guidance on conducting both the systematic review and the meta-analysis.

PDA has typical maturational issues both related to PK (route of administration, dose) and PD (maturational tendency to ductus closure)