Question: What’s the most important customer data to help significantly improve marketing decisions?

Looking for marketing experts to answer:

What's the single most important piece of customer data that can help organizations significantly improve their marketing decisions?

Expert Insight

We have found a 4 question customer interview script, using a modified version of Fred Reichheld's "Ultimate Question", consistently provides great market intelligence as well as increased repeat and referral business.  Of the four questions, the second one, "Why do you rate the customers as you did?" is the single most powerful question.  By probing for operationsal details, customers will both tell the company their purchase criteria, as well as relive what they valued in the work done by the company, which is what drives improved relationships and consequently increased repeat business.  
We find most companies do not do this, and so it enables companies to distinguish themselves versus their competition.  If this is of interest, I would be happy to discuss this with you.  Best wishes, Bill