The differences between the pneumatic compression thromboprophylaxis delivered after elective total hip arthroplasties and that was expected were quantified before (49 patients) and after a concerted nursing education program (30 patients) that was designed to ensure maximum compliance and to verify the correct application of the devices. The expected therapy was not delivered to any of the patients monitored. Therapy was delivered only an average of 77.8% of the time during the expected treatment periods. During 99.9% of the expected therapy times, values of key outcomes-related parameters of the therapy delivered to the patients varied by >10% from expected values. These variations were not reduced significantly by medical and nursing education. This variation may be a significant confounding factor in comparatively evaluating thromboembolic disease outcome reports.