I did retrospective chart review on 240 subjects from my previous institute. THe study was on sedation required for bronchoscopy. The procedure used to be done in the procedure room with mild sedation without anesthesia. Now the reviewers are asking for data from the procedure done in the Operative room under anesthesia, for a comparison.

I left the previous institute and have no access to their database. At my current hospital, we have all the bronchoscopies done in the Operative room and can review the patient's chart in a retrospective manner.

My questions are:
1) Can I select subjects for control group from my current institution assuming the control groups will be age-matched, and the indications for the procedure will be similar?
2) Is that going to be a valid comparison if we select case and control subjects from two different institutions?

Please help me to answer those questions.

Regards,
PM