refers to as bridging ‘the gulf of execution’^4 )? How easy is it for the user to
understand what the system does in response (‘the gulf of evaluation’)? (See
figure 9.3).
Figure 9.3: The gulfs of evaluation and execution. (User icon by Simon Child,
computer icon by Alyssa Mahlberg, both from the Noun Project)
Ideally, the user model maps closely onto the design model. But frequently,
the design model may not be a good fit for what the user wants to do, or users
may only partially understand it. If the system doesn’t conform to any prior
expectations, users must develop a new mental model, based on trying to infer
the design model. Users learn about the design model through the interface,
behaviors of the system and documentation – which Norman refers to as the
system image (see figure 9.4).
Figure 9.4: Diagram: the user, system and design model (redrawn from
http://www.jnd.org/dn.mss/design_as_communication.html))
(^4) Chapter 3 ‘Cognitive Engineering’ in ‘User Centered System Design: New
Perspectives on Human-Computer Interaction’, ed. Norman and Draper, Lawrence
Erlbaum Associates, 1986