It's tempting for designers to start by designing individual screens, but time and time again we discover that it's the tasks and flows that are more fundamental. So whenever we start a new design project, we always set the scope to a specific set of user stories.
In engineering speak: our stories are like unit tests and our screens are like code. The stories exercise our screens and make sure they support all the tasks that the user needs to achieve.
For more about using stories in design: http://gv.com/1759
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