The sample design consists of 24 different choice sets in the CE part. Each
questionnaire contained eight choice questions. To counteract an ordering bias with
the CV part of the survey, half of the questionnaires start with the CV questions and
half of them with the CE part. Figure 2 shows a sample CE question.
Choice design efficiency is determined by level balance and orthogonality (for the
design structure across choice sets), as well as minimal level overlap and utility
balance (for the design structure within choice sets). Level balance means that the
levels of an attribute occur equally often (Zwerina 1997). A level balanced design is
regarded as important. If one level occurs more often than another, this could
influence subjects’ choices when they try to read something into this phenomenon
(Kuhfeld et al. 1994). Moreover, unbalanced design leads to attribute correlation
with the intercepts and a differing statistical power across attributes (Lusk and
Norwood 2005). The choice sets in our survey are level balanced both within the
three subsamples and over all 24 choice sets. Because of the relatively small number
of selected attributes and attribute levels, we first applied a full factorial design while