(This dialog is only available for CBC Advanced Design Module or CVA users.)
You set Null Level Prohibitions under Compose | Conjoint Settings | Prohibited Pairs, and then by clicking the Null Level Prohibitions tab.
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 and CVA software lets you specify a minimum and maximum number of null levels that are allowed to appear together within a concept. You must inform the software of which attribute levels are null levels.
Lighthouse Studio 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 (for CBC designs) 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.
Minimum Null Levels Per Concept
Specify a minimum number of null levels that must appear in each concept. The ability of the software to find a feasible design with reasonable efficiency is greatly affected by this setting.
Maximum Null Levels Per Concept
Specify a maximum number of null levels that can appear within the same product concept.
Align Common Attributes on the Same Row
If this box is not checked, then all null levels will be not be displayed on the screen, and attribute level text below null levels will be shifted upward to remove any blank space. If this box is checked, then the null level text will be shown within the product concept, and attribute level text below it will not be shifted upward into its place. If you still want to display blank text on the screen for the null levels, then you should specify the text (the reserved characters in HTML for the space character) as your null level text.