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Table 3 Interview outline

From: Exploring how individuals complete the choice tasks in a discrete choice experiment: an interview study

Short introduction to the current study including a choice task as a warm up exercise to get used to the DCE and thinking aloud
Part 1: Think aloud part (categorization of participants observed decision-making behavior over four choice tasks, no specific questions asked)
   Categorization options
Choice task reading In which manner participants read the choice tasks Attribute-wise
Directly motivating decision
  Whether participants from the prostate cancer-screening cohort read the opt-out option aloud Yes
Interpretation of the risk attributes How participants mentioned the risk attributes Mentioning actual values
Translating levels into ordinal scale
Mentioning and interpreting values
Testing of continuity axiom The number of attributes participants mentioned when motivating their decision for a certain scenario. Participants were marked as acting in accordance with the continuity axiom if they mentioned three or more attributes (i.e. less than the majority of the five included attributes) when motivating their decision One
Three or more
Decision strategy The decision strategies participants applied to make their decision Traded off attribute levels
Based decision on one attribute
Part 2: Interview part (asking direct questions)
  Questions asked Answer categories
Interpretation of the risk attributes [a] was one of the characteristics that was included in the choice tasks. What did you have in mind with respect to this characteristic when you completed the choice tasks? Exact definition
Other definition
Understanding of the risk attributes Please look at choice task x. If you were asked to make a choice based on [a] only, which scenario would you choose? This question was asked twice for all tested risk attributes (seea). Scenario 1
Scenario 2
Don’t know
  Control question: Participants were asked to make a simple calculation with respect to the risk attributes to test their understanding of the numerical values of the risk attributes. For the rotavirus cohort: ‘Imagine, 1.000 children will get vaccinated with a vaccine that is 95 % effective. Assume that all children will get in contact with the virus. How many children will not get sick?’, and ‘Imagine, 300.000 children will get the rotavirus vaccine. Assume that the vaccine will lead to severe side effects in 1 out of every 100.000 children. How many children will suffer from severe side effects? For the prostate cancer-screening cohort: ‘Imagine a screening program in which out of 1.000 treatments, 200 are unnecessary. Imagine that 2.000 men participate in this screening program. How many men will be treated unnecessarily?’ Right answer
Wrong answer
Don’t know
Testing of continuity axiom Those participants that based their decision on less than three attributes in all choice tasks were asked: ‘You included only x out of five characteristics when making your choice. Why was this the case? Only one or two attributes important
Hard to trade off multiple attributes
Lack of attribute understanding
Part 3: Measuring health literacyb
Subjective health literacy Set of Brief Screening Questions (SBSQ-D) of Chew for prostate cancer-screening cohort only.
This instrument was already included in the initial rotavirus DCE and was therefore not repeated in the current study.
Objective health literacy Newest Vital Sign (NVS-D)
  1. aFor the rotavirus cohort, these questions were asked for both the attributes vaccine effectiveness and frequency of severe side effects, while for the prostate cancer-screening cohort, these questions were only asked for the unnecessary treatment attribute since the levels of the two selected risk attributes (unnecessary treatment and unnecessary biopsy) were considered to be equal. bSee Additional file 3 for more information on these instruments