Varying choice set sizes in discrete choice experiments

Whereas the number of alternatives per choice set in a labeled discrete choice experiment is often determined by the number of available labels, the choice set size in unlabeled choice experiments can be set more freely by the researcher. Determining the number of alternatives that will both yield enough information about the preferences and not overload the choice task for the respondents is, however, not an easy task. If the number of choice sets is restricted, the statistical efficiency of the designed experiment can be increased by increasing the number of alternatives per choice set. On the other hand, large choice sets are complex to deal with and could therefore lead to early fatigue and/or a plethora of screening heuristics that are hard to model. Moreover, although there is no compelling reason to keep the choice set size fixed in unlabeled discrete choice experiments, designs with varying choice set sizes have scarcely been studied. In this paper, we compute and investigate efficient designs with varying choice set sizes. We show that such designs can also be very efficient and we conjecture that such choice experiments are less monotonous for the respondents making it more likely that they will remain attentive. We report on two choice experiments that we conducted to check whether this assertion is correct. We compare designs with equal choice set sizes, with increasing choice set sizes and with random choice set sizes. The post-survey questions indicate that varying choice set sizes are indeed appreciated by the respondents while not reducing the statistical information obtained.