Once a customer tells you he or she is a detractor, It's obviously important to understand why--and to find out in a reasonably efficient, cost effective way. How much of that information can be extracted from the open ended "why" answers customers provide on NPS surveys? It's a daunting challenge. Customers aren't always precise, or complete in their responses, for example.
Satmetrix Director of Methodology Dr. Vince Nowinski provided some in-depth insights. The Stanford PhD described how "comment intelligence analysis" methods allow you to make conceptual sense of such data, to see with greater specififity where problems are actually occuring and what the underlying issue may really be.
But users of NPS much be careful not to look at verbatim analyis as the end all. Rather, my understanding is that it helps you formulate intelligent hypotheses about where the critical issues with detractors lie. Further analysis will typically be required, perhaps in the form of focus groups, follow on surveys to targeted groups, individual interviews etc before you actually get at root causes of the issues detractors have.
Think of it this way. Without verbatims, you could only guess as to what's bothering detractors. With verbatims and the analytical tools described by Vince, you can formulate a very good set of hypotheses for further testing.
