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Commercializing Great Products with Design for Six Sigma
While this text is primarily concerned with the generation of software for commercial sale/use, the sections on preparing and conducting Guided Interviews are very appropriate.
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The text recommends teams of three conduct the interviews. Team leaders actually asks most of the questions, one team member is the scribe to catch verbatim the customer's responses and the third team member is an observer/asks additional follow-up/probing questions.