title: The effect of including an opt-out option in discrete choice experiments.
abstract: OBJECTIVE
  to determine to what extent the inclusion of an opt-out option in a DCE may have an effect on choice behaviour and therefore might influence the attribute level estimates, the relative importance of the attributes and calculated trade-offs.
  
  
  METHODS
  781 Dutch Type 2 Diabetes Mellitus patients completed a questionnaire containing nine choice tasks with an opt-out option and nice forced choice tasks. Mixed-logit models were used to estimate the relative importance of the five lifestyle program related attributes that were included. Willingness to pay (WTP) values were calculated and it was tested whether results differed between respondents who answered the choice tasks with an opt-out option in the first or second part of the questionnaire.
  
  
  RESULTS
  21.4% of the respondents always opted out. Respondents who were given the opt-out option in the first part of the questionnaire as well as lower educated respondents significantly more often opted out. For both the forced and unforced choice model, different attributes showed significant estimates, the relative importance of the attributes was equal. However, due to differences in relative importance weights, the WTP values for the PA schedule differed significantly between both datasets.
  
  
  CONCLUSIONS
  Results show differences in opting out based on the location of the opt-out option and respondents' educational level; this resulted in small differences between the forced and unforced choice model. Since respondents seem to learn from answering forced choice tasks, a dual response design might result in higher data quality compared to offering a direct opt-out option. Future research should empirically explore how choice sets should be presented to make them as easy and less complex as possible in order to reduce the proportion of respondents that opts-out due to choice task complexity. Moreover, future research should debrief respondents to examine the reasons for choosing the opt-out alternative.
authors.affiliation: Centre for Nutrition, Prevention and Health Services, National Institute for Public Health and the Environment, Bilthoven, The Netherlands; Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Jorien
authors.initials: J
authors.lastname: Veldwijk
authors.affiliation: Centre for Nutrition, Prevention and Health Services, National Institute for Public Health and the Environment, Bilthoven, The Netherlands.
authors.forename: Mattijs S
authors.initials: MS
authors.lastname: Lambooij
authors.affiliation: Department of Public Health, Erasmus MC, University Medical Center, Rotterdam, The Netherlands.
authors.forename: Esther W
authors.initials: EW
authors.lastname: de Bekker-Grob
authors.affiliation: Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: Henriëtte A
authors.initials: HA
authors.lastname: Smit
authors.affiliation: Centre for Nutrition, Prevention and Health Services, National Institute for Public Health and the Environment, Bilthoven, The Netherlands; Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands.
authors.forename: G Ardine
authors.initials: GA
authors.lastname: de Wit
mesh_terms: D000328:Adult; D000368:Aged; D002755:Choice Behavior; D003924:Diabetes Mellitus, Type 2 / Q000523:psychology / Q000628:therapy; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009426:Netherlands; D011795:Surveys and Questionnaires