Stability of Willingness to Pay: does time and treatment allocation in a Randomised Controlled Trial influence willingness to pay?

Background
Willingness To Pay (WTP) estimates are only useful to policymakers if they are generalisable beyond the moment when they are collected. To understand the ‘shelf-life’ of preference estimates, preference stability needs be tested over substantial periods of time.
Methods
We test the stability of WTP for preventative dental care (scale and polish) using a payment card Contingent Valuation question administered to 909 randomised controlled trial participants at four time points: baseline (pre-randomisation) and at annual intervals for three years. Trial participants were regular attenders at a NHS dental practices. Participants were randomly offered different frequencies (intensities) of scale polish (no scale and polish, one scale and polish per year, two scale and polishes per year). We also examine whether treatment allocation to these different treatment intensities influences the stability of WTP. Interval regression methods were used to test for changes in WTP over time whilst controlling for changes in two determinants of WTP. Individual level changes were also examined as well as the WTP function over time.
Results
We find that at the aggregate level mean WTP values are stable over time. The results were similar by trial arm. Individuals allocated to the arm with the highest scale and polish intensity (two per year) had a slight increase in WTP towards the latter part of the trial. There is considerable variation at the individual level. The WTP function is stable over time.
Conclusions
The payment card contingent valuation method can produce stable WTP values in health over time. Future research should explore the generalisability of these results in other populations, for less familiar health care services and using alternative elicitation methods.