Effects of helicopter service on survival after trauma. Design of study predisposed to type II error.

EDITOR,--In their paper on the London helicopter emergency medical service J P Nicholls and colleagues, of the University of Sheffield's medical care research unit, mention several concerns1 that we expressed at the time that the unit published its first report on the service to the Department of Health in 1994.2 The helicopter is part of an integrated specialist trauma service, and it is unfortunate that the unit looked at parts of the service separately. In doing so the unit isolated the effect of the helicopter from the effect of the hospital. We …