Design of Everyday Things [100]
CHAPTER SEVEN
USER-CENTERED DESIGN
OFF THE LEASH By W.B. Park
“Darn these hooves! I hit the wrong switch again! Who designs these instrument panels, raccoons?”
The point of POET is to advocate a user-centered design, a philosophy based on the needs and interests of the user, with an emphasis on making products usable and understandable. In this chapter I summarize the main principles, discuss some implications, and offer suggestions for the design of everyday things.
Design should:• Make it easy to determine what actions are possible at any moment (make use of constraints).
• Make things visible, including the conceptual model of the system, the alternative actions, and the results of actions.
• Make it easy to evaluate the current state of the system.
• Follow natural mappings between intentions and the required actions ; between actions and the resulting effect; and between the information that is visible and the interpretation of the system state.
In other words, make sure that (1) the user can figure out what to do, and (2) the user can tell what is going on.
Design should make use of the natural properties of people and of the world: it should exploit natural relationships and natural constraints. As much as possible, it should operate without instructions or labels. Any necessary instruction or training should be needed only once; with each explanation the person should be able to say, “Of course,” or “Yes, I see.” A simple explanation will suffice if there is reason to the design, if everything has its place and its function, and if the outcomes of actions are visible. If the explanation leads the person to think or say, “How am I going to remember that?” the design has failed.
Seven Principles for Transforming Difficult Tasks into Simple Ones
How does the designer go about the task? As I’ve argued in POET, the principles of design are straightforward.1. Use both knowledge in the world and knowledge in the head.
2. Simplify the structure of tasks.
3. Make things visible: bridge the gulfs of Execution and Evaluation.
4. Get the mappings right.
5. Exploit the power of constraints, both natural and artificial.
6. Design for error.
7. When all else fails, standardize.
USE BOTH KNOWLEDGE IN THE WORLD AND KNOWLEDGE IN THE HEAD
I have argued that people learn better and feel more comfortable when the knowledge required for a task is available externally—either explicit in the world or readily derived through constraints. But knowledge in the world is useful only if there is a natural, easily interpreted relationship between that knowledge and the information it is intended to convey about possible actions and outcomes.
Note, however, that when a user is able to internalize the required knowledge—that is, to get it into the head—performance can be faster and more efficient. Therefore, the design should not impede action, especially for those well-practiced, experienced users who have internalized the knowledge. It should be easy to go back and forth, to combine the knowledge in the head with that in the world. Let whichever is more readily available at the moment be used without interfering with the other, and allow for mutual support.
THREE CONCEPTUAL MODELS
The operation of any device—whether it be a can opener, a power generating plant, or a computer system—is learned more readily, and the problems are tracked down more accurately and easily, if the user has a good conceptual model. This requires that the principles of operation be observable, that all actions be consistent with the conceptual model, and that the visible parts of the device reflect the current state of the device in a way consistent with that model. The designer must develop a conceptual model that is appropriate for the user, that captures the important parts of the operation of the device, and that is understandable by the user.
Three different aspects of mental models must be