5. How should a customer answer a non-relevant Preference Interview question?

A customer should answer '1' for their importance score for non-relevant questions.

Most of the time, all the questions in a Preference Interview or a Preference Survey should be relevant to every customer. However, occasionally, there are circumstances where a question will not be relevant. 

When this happens, instruct the customer to answer a '1' for Importance and a '10' for Satisfaction. Even without these instructions, these are the responses one would make for non-relevant questions.

Later, when creating Market Views, you should create sub-segments of customers with similar values. 

As an aside, some have asked, "Would it be better to have an N/A or 'Not Applicable' response?" And no, this would not be better. It can make it too easy for some customers to select this option, and they may not be thinking about every context. And further, this could create arbitrary differences where some select "N/A" and others select "1" for Importance.