Design Principles
(Presented by D. Norman in The Design of Everyday Things)
Visibility
Make relevant features and their controls clearly visible to the user.
Examples:
- Putting the "Order" button on the company's Web page catalog.
- Hiding special features on a photocopier control panel in order to reduce
clutter and make more frequently used controls more visible.
Mappings
Take advantage of the natural correspondence between objects in the world
and object in the user interface. The mapping can be physical analogies
(e.g., moving a control up to move an object up) or cultural standards (e.g.,
moving a control up to turn power on).
Example:
- The Mercedes seat control (p.24),
User's Mental Model
Consider how the user might interpret (or misinterpret) the invisible mechanisms
or functional logic of a system.
Examples:
- Norman's refrigerator controls.
- When a computer user sends a job to a printer does the user really understand
that the job is submitted to a queue on a print server?
Affordance
Properties of an object afford specific actions. As a designer, keep such
affordances in mind and select objects which are appropriate for the task.
Examples:
- Buttons afford pushing.
- Knobs afford turning.
Constraints
Use physical, cultural, logical, and semantic factors to limit the alternative
actions a user can perform on an object, thus making its use more obvious.
Examples:
- Assembly can be aided by physically prohibiting incorrect arrangement
of parts.
- Levers that only move along one dimension.
Memory
Reduce the memory load on the user by providing the information (in the
world) at the appropriate place and time.
Examples:
- Rubber gloves provided on instruction sheet with hazardous aerosol product.
- Pictures rather than arbitrary codes on grocery scales.
- Graphical representation of stored objects rather than representation
by alphanumeric names.
Feedback
Provide obvious, immediate, and relevant feedback for user actions.
Examples:
- An audible snap when plastic parts are correctly assembled.
- A two position switch which stays in a visibly different position after
being thrown.
- A light on the VCR signaling that the it is in play mode even though
the image has not yet come on the screen.
Iterative Design
Initial designs are seldom successful. Expect to design, test, and redesign
a product several times before getting an acceptable solution.
Usability Testing
A method for testing designs before paying customers do. A logical way
to get through the design iterations mentioned above.
Paradox of Technology
Added functionality often leads to decreased usability of a product. Leaving
features off a product make it easier to use. Trouble often results when
the number of functions exceeds the number of controls and available displays.
Examples:
- Photocopiers that staple, fold, fax, scan, print, collate, etc.
- Cockpits with more functions than controls and displays.
Design Tradeoffs
Usability is usually not the only (and often not the most important) aspect
of a product from the purchaser's perspective. Increased usability often
increases cost or has some other undesired effect. Every design is a compromise.
Example:
- I recently purchased a compact stereo. I opted for the one with better
sound quality over the one that was easier to operate.
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