CVA can use the a priori preference relationships (i.e. "best to worst") you specified for your attributes to avoid presenting conjoint profiles that would result in obvious answers. For example, questions including a product that has many good features at a low price or many bad features at a high price may be less useful for deriving conjoint utilities. Questionnaires that avoid the most obvious comparisons are more challenging for respondents and usually provide more information. A moderate degree of balance in the expected utility of the concepts in CVA questionnaires is therefore desirable. However, we caution that too much balance can increase the difficulty of the questionnaire and the error in the responses, counteracting the benefits of improved utility balance.
CVA's method of discarding "obvious" tasks takes a conservative approach that eliminates only the most extreme cases. Advanced researchers can further balance the design by specifying additional prohibitions. This may be a reasonable approach so long as the researcher conducts substantial quantitative pretests to ensure that the benefits from utility balance are not counteracted by increased response error.
Recall that pairwise designs show two product concepts per screen. For each attribute with a priori order information, if the concept on the left has a higher utility than the one on the right, a +1 is scored; if it has a lower utility, then it is scored -1. For single-concept designs, only extreme levels are counted. Highest utility levels are scored +1, and lowest levels -1. Non-a priori attributes are ignored, because CVA has no way of knowing which level is preferred. If the imbalance exceeds some threshold, then the task is considered "obvious" and discarded. The threshold used to determine "obvious" tasks is >75% of a priori attributes marked as +1.
The following table below shows the threshold used for each number of attributes (up to 10) to determine "obvious" tasks:
"Obvious" Tasks Threshold
For example, for studies involving just 4 attributes, the threshold score must reach 3 before the task is considered "obvious."
Because a modest degree of utility balance is a desirable quality, it may make sense in the design phase to set any attribute to a priori for which you have a reasonably good idea about which levels are preferred to others. For example, if some brands are generally preferred to others, you might specify them in order from generally least to most preferred and then set the a priori setting to "worst to best." If you take this approach, make sure to remove any a priori setting for all attributes that do not have a clear order of preference before calculating utilities (under Additional Utility Constraints), or you may impose constraints that are not consistent with some respondents' preferences.