Irrelevant people give irrelevant answers.
A range of products tested in a study is often too broad for an individual to be interested in all of them. Individuals differ in consideration sets by their needs, expectations, budgets and alike. So that an interview is efficient and unbiased, the selection of tested products and the related questions should aim at the respondent's consideration scope. Only a product acceptable for the respondent can be chosen with a sufficient reliability. Therefore, the design of a conjoint exercise should allow generation of choice sets given the respondent, and allow to build the situations in which decisions are supposed to be made.
The danger of asking about profiles outside the consideration scope lies in the phenomenon known as impersonation. Instead of refusing profiles not fitting their needs and expectations, people tend to adapt and shift their personal attitudes from their own consideration scope into the frame of the offered profiles. The answers are correct, but do not correspond to consumption habits of the interviewee. A person living mostly on hamburgers and donuts can easily distinguish rise from beens or caviar from lobster, and state his or her preferences but will hardly change habits and contribute to the consumption of the inquired goods.
A common approach to the problem is to
split a sample into segments with help of filter questions, and for
each segment have a separate version of the questionnaire such as a
conjoint block, concept test, battery of question,
etc. There are situations where this strategy does not work well
because the filter questions cannot be
asked jointly. E.g., in case of testing complex products with many
attributes, price categories or schemes, component parts of
bundles or packages, marketing ideas, etc., formulation of proper
filter questions may be
A short pseudo-conjoint test with a limited number of items that represent the whole range of the studied items can be designed. Every respondent is presented with a set of product profiles (at best about 12 to 24, but 36 relatively simple profiles have been verified as feasible) where the profiles represent both the reference (usually the current) and the expected state of the market. The set is composed either of ad-hoc profiles reasonably selected from some pool, or, preferably, of items satisfying the rules of efficient design, possibly using a product classes approach. The responses to the stimuli are analyzed in course of the interview, the range of interests is determined, and the respondent is assigned the block of the questionnaire respective to his or her interests. The assignment is based on the sampling scheme known as "independent multinomial sampling" or "product multinomial sampling". The procedure falls to a group of the CBS - Choice Based Sampling methods.
|Compared to standard filter questions, CBS has some advantages.|
The SCE -
Choice Exercise, namely a version with removal of unacceptable items in
the first step, has proved useful as
Only several first
choices from the set are needed to assign one of the predetermined
questionnaire variants to the respondent.
The assignment is based on finding
highest inclusive value of the chosen selection as projected onto the
the questionnaire variants.
|The set of profiles used in CBS should respect the following requirements:|
In connection with assignment of a conjoint block to a respondent, it is of advantage to represent each class by 3 or 4 representative profiles. With the total number of 12 to 21 profiles, 3 to 7 classes of products can be represented in the set. To make a balanced representation of a class, three-level benefits (with the levels "Low", "Medium" and "High") of the 3 most influential attributes (A#1, A#2 and A#3) may be set according to the following table.
||1st Concept||2nd Concept||3rd Concept|| 4th Concept
The level "high" should be the highest level in the class this
sub-block points to. All other attributes should be set so as to
achieve the most
realistic product profiles. The design in the table is balanced
both in theoretical and practical way.
Outer levels are
shown with the same frequency and all profiles have medium
attractiveness on average.