The goal of CBC design is to gather enough data from each respondent to estimate their preferences accurately, without overwhelming them.
To help, Discover includes a built-in recommender that looks at your defined attributes and levels and suggests the number of tasks and concepts per task. If you’d like more control, you can override these recommendations in the Exercise design settings (found in the Advanced tab of a CBC exercise).
When the Exercise design override toggle is switched off (input fields are disabled), a CBC exercise’s settings (such as number of tasks and how many concepts to display per task) are auto-generated based on the number of list items included in the exercise.
“Tasks” are also known as questions, concepts, profiles, or cards.
Even in the disabled state, however, the fields in this section communicate valuable information about the exercise design that will be used during data collection (fielding). You can see the total number of tasks (questions) a respondent will be shown as well as the number of concepts (profiles or cards) that will be shown in each task.
Behind the scenes, the Discover exercise designer selects attribute levels to show across multiple tasks (questions) according to the following goals:
Depending on your attribute/level list and number of tasks (questions) to show, it may not be possible to achieve perfection on all three goals; but CBC questionnaires that are almost-but-not-quite perfect are still very efficient and you will obtain excellent results in practice.
If the default exercise designer makes design recommendations that you disagree with, or you simply want to have control over the design parameters, then use the override toggle to enable the design settings input fields.
Switch the toggle back to the off position and the settings will revert to the default recommendation.
Notice that when modifying the Number of concepts per task, the recommendation shown to the right of the Number of CBC tasks input field changes.
The Number of CBC tasks setting configures the number of tasks (individual questions within the exercise) that the respondent must answer.
It is possible that Discover will recommend a design that has a high CBC task count. In general, we don’t recommend that you decrease the Number of CBC tasks because this reduces the information available to estimate accurate preference scores.
If the recommended range for Number of CBC tasks exceeds what you think respondents can answer without getting tired and giving you bad data, then you should consider the following:
Based on the largest number of levels in any one of your attributes, the Discover designer recommends the Number of concepts per task (excluding the "None" concept).
From a statistical viewpoint, showing more Concepts per task is better for obtaining precise preference scores. However, showing more concepts per task makes it harder for humans to see the information on smaller screens and accurately complete the CBC exercise.
We generally suggest presenting at least three Concepts per task (in addition to the none option). However, in certain cases such as emotionally laden decisions involving different cancer treatments, it may be extremely difficult for respondents to consider more than two concepts at a time.
During CBC exercise design configuration in Discover you may run into warnings or errors regarding the settings that you have chosen. See the following for explanations of these messages:
If you have provided a lot of attributes or levels, the designer may suggest a Number of CBC tasks that is difficult for respondents to adequately complete. Although the recommendation may be high, the warning message is still shown because that task count is needed to produce accurate utility scores for each respondent. Consider reducing the number of attributes and levels to bring down the recommendation.
If you have ample sample size and are willing to accept more noise at the individual level than is typical for conjoint analysis, you may proceed with a sparse design featuring fewer tasks than recommended.
Since 30 tasks is likely difficult for a respondent to accurately complete, the software gives a warning when you adjust the design to require more than this. You may choose to still use this design but proceed with caution.
At least one task must be shown to respondents to field a CBC exercise.
It may be confusing to respondents if there are concepts that look exactly the same in a single task. The software alerts you when you will no longer be able to have unique concepts in the same task.
At least two concepts (aside from the "none") must be shown to the respondents in Discover.
Requiring respondents to compare many concepts may be taxing and could lead to poor utility scores. It is also difficult to view on small screens.
A CBC exercise cannot include more than 100 concepts in a single task.
A CBC exercise cannot include more than 100 tasks.
