This dialog lets you specify which combinations of attribute levels (or attributes altogether) may not be shown together within the same product concept in conjoint questions.
For standard (within-concept) prohibitions, select an attribute in the upper pane and a second attribute in the lower pane. A grid appears, in which you can check which level combinations should be prohibited.
For advanced prohibitions for CBC studies (described below), select either the Advanced Prohibitions or Null Level Prohibitions tabs (requires license to the Advanced Design Module).
Prohibitions, if at all possible, should be avoided. Specifying unnecessary or excessive prohibitions is one of the most common mistakes. The problem usually begins when either the analyst (or the analyst's client) notices that some product combinations displayed during the interview are not realistic, given what currently exists in the market. Sometimes a product is shown with all the best features at the lowest price; or two attribute levels that would not naturally occur in the real world are paired together. The inclination is simply to prohibit such combinations. We urge you to exercise restraint when considering prohibiting pairs.
Full-profile CBC studies are particularly sensitive to attribute prohibitions. Too many prohibitions can lead to imprecise part-worth estimation. It is better to prompt respondents that they will see combinations during the interview that are not yet available in the market or that seem unlikely. You can urge respondents to answer as if these products were actually available today.
If you use any prohibitions, you must make sure to test your design and check the efficiency of the estimates for the parameters of interest.
How Many Prohibitions Can I Safely Add?
A common question that users ask is "how many prohibitions can I specify without seriously damaging the results of my study?" This cannot be answered without more information. It is not the sheer number of prohibitions but the specific pattern of prohibitions that more directly affects the degree of correlation among the attribute levels in the design matrix and thus the design efficiency.
Assume that the researcher wants to specify 3 prohibitions between a 2-level attribute and a 3-level attribute. There are just 6 possible combinations that can occur when combining those two attributes. If the researcher prohibits 3 of those combinations from occurring, this eliminates half of the possible combinations between those attributes. There would be less damage to the efficiency of the design if three total prohibitions were allocated across three separate attribute combinations (one per pair of attributes).
As a side note, the researcher in this instance may decide to specify the three non-prohibited combinations of those two attributes as a single attribute, rather than as two separate (and not independent) attributes. The main drawback of this approach is that after combining these two attributes, the researcher will not be able to compute the attribute importance or the relative part-worths of each attribute independently.
For many years, our CBC software only supported within-concept prohibitions between the levels of pairs of attributes. In other words, within the same product concept, one level of one attribute would be prohibited from appearing with one level of another attribute. For example, we would prohibit Brand A from appearing with the color Green within the same product concept. Starting in v6 of CBC, we began to support the following advanced prohibitions:
You can define prohibitions that simultaneously involve up to n attributes, where n is the number of attributes in your study. For example, a 3-way prohibition involves prohibiting the combination [Brand A, Style C, Green] from appearing (where brand, style, and color are three separate attributes).
You can define prohibitions restricting what attribute combinations may occur in competition with another within the same choice task. For example, we may wish to prohibit a [Brand A, Style C] product from being shown within the same choice task in competition with a [Brand A, Style B] product. These may be defined as 1-way to n-way prohibitions.
Some researchers include "blank" levels within attributes (often by using the HTML syntax for the attribute level text, which displays a blank space on the screen). The null (blank) level reflects the absence of the attribute. However, if many attributes have null levels, the analyst may wish to limit how many of them appear together in their null state. CBC software lets you specify a minimum and maximum number of null levels that are allowed to appear together within a concept (though specifying that the number of null levels permitted should be constant leads to confounded designs and is not permitted). You must inform the software of which attribute levels are null levels.
As you place constraints on the design to enforce a narrow range of permitted null levels per concept, the design efficiency is reduced. Furthermore, the greater the null level constraints, the worse that Shortcut, Complete Enumeration, and Balanced Overlap methods will perform relative to the purely Random method. With quite constrained solutions, the Random design method performs best. For this reason, it is very important that you test different design methods and compare the overall efficiency of the designs when using null-level prohibitions.
Effect of Prohibitions on Design Efficiency
Within-concept prohibitions are generally more damaging to design efficiency (the ability of the questionnaire to estimate the part-worth utility parameters with good precision) than between-concept prohibitions. A higher-order prohibition (for example, a single 3-way, within-concept prohibition) is less damaging to the precision of utility level estimates than a 2-way within-concept prohibition. This follows logically, since a single 3-way prohibition leaves many more attribute level combinations available to display than a single 2-way prohibition, which is more restrictive.
Please note that to estimate interactions between two attributes, all possible level combinations of those two attributes must have been displayed within concepts to respondents (across all respondents). Therefore, specifying a 2-way, within-concept prohibition would preclude the ability to estimate an interaction effect between the two attributes involved in that prohibition. However, specifying a single 3-way prohibition would not prohibit estimating 2-way interaction effects (since all 2-way combinations of attribute levels would still be represented).