The 'No-Choice' Alternative in Conjoint Choice Experiments

Conjoint choice designs are frequently applied in practice, and often a base alternative is added to the design. When such a 'no-choice' base alternative is present in conjoint choice experiments a constant term should be added to the design ('X'-) matrix with attribute dummies when effects type and/or linear coding is used for the attribute levels. Not including such a constant may result in a much lower model and predictive fit and even biased estimates for the (linear) attributes