Order bias in estimates of willingness to pay for drugs to treat attention-deficit/hyperactivity disorder.

ABSTRACT

This study investigated the presence of order bias in contingent valuation studies. We compared the willingness to pay (WTP) for an improved drug for attention-deficit/hyperactivity disorder with that for the standard drug and examined whether the order in which the drugs are presented influences the results obtained. A pretested questionnaire was administered to 114 university students. WTP was elicited using the payment card method. A split sample was used to test order bias. The average WTP per month was euro 57.34 for the standard drug and euro 81.95 for the new drug. The results show that WTP for a new drug varies according to the order in which the drugs are presented. Respondents stated a higher WTP for the new drug if the standard therapy was presented first. This may be due to order bias, evidence of which has also been found in other studies and which is considered to be a limitation of the method. Finally, the use of marginal WTP is suggested as a solution.