title: Causal Inference in Observational Studies in Surgery.
authors.affiliation: Clinical Surgery University of Edinburgh Royal Infirmary of Edinburgh Edinburgh, UK ewen.harrison@ed.ac.uk.
authors.forename: Ewen M
authors.initials: EM
authors.lastname: Harrison
authors.forename: Stephen
authors.initials: S
authors.lastname: OʼNeill
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Wigmore
authors.forename: O James
authors.initials: OJ
authors.lastname: Garden
mesh_terms: D041881:Cholecystitis, Acute / Q000601:surgery*; D002767:Cholecystostomy / Q000295:instrumentation*; D005260:Female; D006801:Humans; D008297:Male