The Effect of Differences in the Number of Attribute Levels in Conjoint Results

It is well known that the range of attribute variation used in a conjoint design influences the inferred attribute importance. However, even if the range is held constant, the addition of intermediate levels can increase this importance. In this paper we show why the problem occurs for rank-order preferences. The results from an experimental study confirm the existence of a systematic influence due to the number of (intermediate) levels. Surprisingly, the problem is equally strong when rating scale preferences are collected. Several possible solutions are suggested.