Is stated preference certainty individual-specific? - An empirical study

The somewhat ad-hoc method of certainty calibration, based on self-stated preference certainty follow-up questions, has been found to be a successful method of eliminating or reducing hypothetical bias in stated preference studies. But is the preference certainty really context dependent, or do some subjects tend to always state themselves as certain regardless of the context, i.e. is the preference certainty dependent on a systematic unobservable individual-specific effect? This question is empirically analyzed in this paper using data where a preference certainty question follows a hypothetical willingness to pay question, in two different contexts. Estimated bivariate probit models provide no evidence for systematic individual-specific answers to the preference certainty follow-up questions of different contexts. Since there is no support for a randomly self-stated preference certainty either, this result is deemed to increase the credibility of certainty calibration.