To investigate the issue of conjoint reliability over time.
A discrete choice experiment was applied using scenarios that describe the effect of treating rheumatoid arthritis patients with TNF-alpha inhibitors, a novel class of highly effective, but expensive antirheumatic agents. Respondents participated in three face-to-face interviews over a period of 4 months. Reliability was measured both at the input level, where the consistency of matches made by respondents to the Discrete Choice Experiment (DCE) question between replications was determined, and at the output level, where the parameters of the conjoint model were estimated and tested for joint significance and willingness to pay (WTP) confidence intervals were calculated.
Input level: Of the 1661 choices made in survey 1, 1316 were repeated in survey 2. Based on the observed number of consistently repeated choices and the expected number by chance, a fair agreement between the choices in the two surveys (chi2 = 324) was found. Of the 998 consistently repeated choices from survey 1 to survey 2, 818 were repeated in survey 3. There was again a high level of consistency between the choices in surveys 1 and 2 and the final choice in survey 3. Output level: The confidence intervals for WTP figures in surveys 1 and 2 and 1 and 3 were overlapping, implying that the DCE was reliable at the output level over time.
The proportion of consistent responses was higher than would be expected by chance. Conjoint reliability over time was found both at the input and output level.