Don Norman's jnd.org / Ad-Hoc Personas & Empathetic Focus. AD-HOC PERSONAS and Empathetic Focus (revised 16 Nov. 2004) John Pruitt and Tamara Adlin are writing a book on the use of "Personas" in design.
They asked if I would write a sidebar to be inserted into the relevant chapter of their book: here it is, but with additional notes and discussion. To be printed in Pruitt, J, & Adlin, T, "The Persona Lifecycle: Keeping people in mind during product design. " Discount Usability: 20 Years (Jakob Nielsen's Alertbox) AgileManagement.