Braithwaite 1998.

Methods	Retrospective cohort, ground comparison group only
Participants	Trauma patients in the Pennsylvania Trauma Outcome study, 1978‐1995, USA
Interventions	HEMS versus GEMS transport to trauma centres
Outcomes	Mortality
Notes	Not all hospitals were level I trauma centres, types of injuries not described
Risk of bias
Bias	Authors' judgement	Support for judgement
Reporting bias	High risk	5/11
External validity	Unclear risk	2/3
Internal validity	High risk	4/7
Internal validity: confounding and selection bias	High risk	3/6
Power	High risk	0/5