Design Guidelines for Control Functions and Behavior
4/8/2010
The following factors should be considered when designing device controls and behavior associated with them.
Placement and Durability
Proper and reasonable control placement is crucial to ease of use. Control placement should be prioritized in a way that reflects their use. High–use controls (such as navigation) should occupy premium space on the hardware. Lower priority controls should be also placed in easily accessible locations even if they are not in premium locations.
Control switches, keys (the surface touched by the finger) and legends should be durable enough to withstand regular use over a reasonable period of time. During this period of time, legends, keys and tactility should not degrade to unacceptable levels, as established by existing durability and reliability standards.
Discoverability
Many device controls are familiar to most users, such as the navigation area, d-pad, the 12 key area, etc. Some controls are often a guessing game as to their function. These are usually the dedicated hardware controls that map to functions like voice recorder, camera operation, etc. These controls are often poorly labeled or sometimes not labeled at all. Device designers should strive to create meaningful labels, testing them with real users, such as usability study participants not familiar with the device, to see if the icons or labels or legends are helpful.
It is true that it usually requires very few operations of unknown controls to learn their functions but many users hesitate to operate controls that are poorly labeled or not labeled out of fear of not being able to recover from unwanted changes or data loss.
Predictability
Buttons and controls should return predictable results. Controls whose results are different and whose differences remain opaque to the user are extremely frustrating. Contextual controls should be implemented in a way that prepares the user for the kind of result they will see, or makes it clear to them in some way the change in control function associated with the new context.
Controls should also be predictable in tactile feedback. Aural changes or changes that return different sounds generated by the device based on context are an acceptable means of conveying state changes or notifications, as long as the aural changes are logical to the user. Usability testing for contextual change is highly recommended.
Overloading Controls
Overloading controls is the practice of associating more than one function with one control or button. An example of this is pushing and holding the End button to turn a device on and off. With the exception of the most common overloading of the End button to turn a device off, any overloaded keys functionality is extremely hard to discover. Users often will not refer to included documentation unless absolutely necessary. You should expect the functionality of most overloaded controls to be undiscoverable. For this reason, hardware designers are encouraged to keep overloaded keys on the device to a minimum.