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The interview guide should include a single sentence purpose statement. The book gives several sample templates for the structure of these sentences but they generally breakdown into:
To Verb for gaining information Customer Focus in order to Business Focus
A simple example of this from my own recent projects would be:
To discover the methods and standard practices that the MRC uses to provide AV and technical support to events in order to help design a system which will automate and track equipment/inventory as well as provide additional billing capabilities.