The estimate of benefit is likely to be internally valid. However, the lower incidence of DVT found in the study compared with the literature (4.6 to 15%) may, as noted by the authors, be attributable to the aggressive enforcement of prophylactic measures as part of the critical path followed by medical staff. The study also had a high number of patients who were able to receive SCD or SH or both, but the literature suggested this was rather higher than is normally the case.