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doi:10.1136/bmj.311.7013.1166

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Jon Nicholl,
Helen Snooks,
John Brazier

Director Research associate Senior lecturer Medical Care Research Unit, University of Sheffield, Sheffield S1 4DA

EDITOR,--As D W Maclean and colleagues point out, we had hoped to isolate the effect of the helicopter and team from the effects of the major developments at the Royal London Hospital as well as assessing the effect of the helicopter emergency medical service as a whole. We were unable to do this with any reasonable power because too few control patients (those attended by an ambulance) who met our inclusion criteria were taken to the hospital during the 21 months of the study (n=40), not …