Attributes and Levels for Adaptive CBC

For the most part, designing attributes and levels for ACBC follows the same guidelines as for other conjoint methods. We'll assume the reader already is acquainted with this practice (refer to the relevant sections in the ACA, CVA, and CBC documentation). But, there are some key differences related to the treatment of price and the possibility of using constructed lists to customize the attribute list each respondent sees.

Most projects will probably present each respondent the full list of attributes and levels in the study. However, ACBC gives you the option to drop attributes and/or levels from consideration (you need to do this prior to the respondent starting the ACBC questions). This is facilitated by SSI Web's constructed list building logic. As you enter your attributes and levels for your ACBC study, you'll be specifying them within Predefined Lists. The attribute list is a Predefined List with as many elements as attributes in your study. Each element in the attribute list is associated with a Predefined List that enumerates the levels.

Generally, we recommend that no more than about 12 attributes with no more than about seven levels each be taken forward to the ACBC questionnaire. If your study involves more attributes and levels than this, you may want to use preliminary questions within SSI Web (CiW) to eliminate any attributes that are completely unimportant and any levels that are completely irrelevant.

Although it is possible to include a Price attribute as an attribute with discrete levels (such as $10, $15, $20), we generally recommend using ACBC's Summed Price approach and to treat price as a continuous variable. This is described in the next section of this documentation.

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Sequence Order and Preference Order

In the "Unacceptables" and "Must Have" questions, we need to understand something about the nature of the levels (sequential order and/or preference order) to phrase our questions correctly and to make correct logical deductions regarding which levels to eliminate from future consideration. For example, assume we are conducting a study on automobiles and one of the attributes is fuel efficiency (in Miles Per Gallon) with levels:

20 MPG

25 MPG

30 MPG

35 MPG

If after the first few screens of product concepts, the respondent only indicates that automobiles with at least 30 MPG are "possibilities," then the correct way to phrase the "Must Have" question would be something like this:

The automobile must feature... "Fuel Efficiency: At least 30 MPG"

For ACBC to know whether to use text such as "at least" or "at most" requires knowledge of whether the attribute has sequential (such as numeric) order, and which direction the order goes. With the fuel efficiency attribute, the sequential progression is from low to high values. For ACBC to know whether to eliminate more extreme levels from consideration if a less extreme level is marked "unacceptable" or "must have" requires understanding if the attribute involves a rational preference order. In this case (all else equal) there is rational preference order from worst to best. So, if the respondent confirms that "at least 30 MPG" is a cutoff rule, then we know that both 20 MPG and 25 MPG are unacceptable.

When defining attributes, you must specify whether each attribute has:

·

Sequential (e.g. numeric) order (low to high or high to low)

·

Preference order (worst to best or best to worst)

The preference order is only used in the questionnaire for determining how to offer and enforce cutoff rules. It is not automatically applied as a utility constraint within estimation. You have the opportunity to enforce monotonicity (utility) constraints separately within the estimation modules.

Here are some examples of attributes and their classifications:

Non-ordered:

Brand

Coke

Pepsi

Sprite

Sequential Order (high to low), Preference Order (Best to Worst):

Printer Speed

40 pages per minute

25 pages per minute

15 pages per minute

10 pages per minute

Sequential Order (low to high), No Preference Order:

Square feet in home

1250 square feet

1750 square feet

2250 square feet

3000 square feet

4000 square feet

5000 square feet

(Note that a person may not prefer a 5,000 square foot home to a 2,250 square foot home, because it involves extra work to clean, and is more expensive to furnish and maintain.)