Perform usability evaluation as early as possible in the design cycle by building and evaluating prototypes.
Prototypes cut down on either the number of features, or the depth of functionality of features:
Vertical Prototype: in-depth functionality for a few selected features.
Horizontal Prototype: full interface features, but no underlying functionality.
Scenario: only features and functionality along a pre-specified scenario (task) or path through the interface.
These varieties of prototype are illustrated in Figure.