The idea is that you can get a lot of feedback about the interaction between the interface and the user by evaluating a low-fidelity prototype. Since low-fidelity prototypes are cheap, both in terms of money and time, you can afford to have more cycles of testing, more subjects, or more prototypes.
Also, make paper representations of menus, button groups, scroll bars, and the like. Post-It notes are good for this, as you can plunk them down on top of your "screen" as the user "clicks" on items, then rip them off as the state of the interface changes.
Some people use transparancies for layering elements on top of one another. I think that's too much trouble. Besides, one of the cool things about low-fidelity prototyping is that you can invent new interface elements during the evaluation session, simply by drawing them onto a Post-it note. Transparancies are a bit messier and harder to work with during the evaluation.
Sometimes the action can get pretty hectic, especially if you have one of those expert users that loves to click everywhere. It helps to have a number of elements pre-drawn, and organized (I've used a chessboard to sort small Post-it note elements in a matrix) so you can easily grab an element when you need it.
Nielsen, Jakob, "Paper versus Computer Implementations as Mockup Scenarios for Heuristic Evaluation'', Human-Computer Interaction-Interact `90, D. Diaper et. al. (ed.) Elsevier Science Publishers B.V. (North Holland), 1990: 315-320
Rettig, Marc, "Prototyping for Tiny Fingers (Everything I Need to Know About Prototyping, I Learned In Kindergarten)'', Communications of the ACM, April 1994.
Rubin, Jeffrey, Handbook of Usability Testing, 1994, John Wiley and Sons, New York, NY ISBN 0-471-59403-2 (paper)
Snyder, Carolyn, "Using Paper Prototypes to Manage Risk," October 1996, Software Design and Publisher Magazine
Virzi, Robert A, Sokolov, Jeff, and Karis, Demetrios.
"Usability Problem Identification Using Both Low- and High-Fidelity Prototypes,''
1995: Obtained directly from the authors.
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