General population versus disease-specific event rate and cost estimates: potential bias for economic appraisals.

Economic appraisals are increasingly being used for reimbursement decision making. Differences exist in the population data sources used in different studies and these differences may result in errors or biased estimates. A review of the literature suggests that very little has been written on this topic and guidelines and good practice documents are silent on the issue. Using illustrative examples, it was found that the population chosen for event/complication costing did not have a large impact on a cost-effectiveness analysis; however, the choice of population did have a large impact for cost-of-illness (COI) estimation. It was found that not controlling for event/complication rates in a nondiseased population resulted in a 15% inflated COI estimate and using event costs from the general population underestimated COI by 20-32%. Our analysis suggests that using event costs from the general population instead of a diseased population may not have a significant impact on cost-effectiveness estimates; however, COI studies should only use excess event/complication rates and should also only use event costs from populations with the disease.