PREPRINT Users , Roles , and Personas

ion and independence of implementation enables the modeler to focus on the essence of the task from the perspective of the user without complicating the picture by jumping ahead into details of the visual and interaction design. In this example, the abstract essence of my task as a user is just to tell the system who I am, make a choice, and get my cash. Essential models challenge the designer both to understand the true nature of the problem from a user perspective and to provide a much simplified solution. Abstraction highlights opportunities for further exploration and for innovation, such as making the choices offered reflect user intentions, such as withdrawing the usual amount, rather than bank policies and systems structures. If I have only one account and always withdraw $240 dollars from it, I should not have to tell the ATM which account and re-enter the amount on every withdrawal, for example. Task models in the form of essential use cases have shown that they can help designers identify innovative solutions that better fit with genuine user needs (Constantine and Lockwood, 1999; 2002; Strope, 2003; Windl, 2002b).