Analyzing the why's of interface features is complicated and detailed, as the example showed. But it's possible to identify some broad kinds of arguments that crop up often.
Small targets are harder (and slower) to hit with a mouse than big targets; long mouse movements are slower than short ones; icons pack differently from text strings; more keystrokes take longer to type; switching between mouse and keyboard is slow.
It is easier to recognize something when you see it (for example on a menu) than it is to recall it from scratch (for example in typing in a command); it is hard to remember much information from one step in a process to another (so, for example, having help information available at the same time as the user carries out an operation is a good idea; more generally, information that is used together should be presented together); the interface should present key information, such as the current mode, rather than requiring the user to remember it.
Interface features should help the user to select operations that are relevant to their goals, by labelling the operations in ways that match the way the user thinks about his or her task; the user needs to know what an operation has actually done (the UNIX approach of saying nothing unless something goes wrong is useless if you are a learner and do not already know what the commands do); users will make mistakes, especially if they are exploring options, so give them a way to back out.
Information presented with a big change in the display is more likely to be read; information presented close to where the user is looking is more likely to be read; auditory signals cannot be ignored as easily as visual signals (this is a two-edged sword; sometimes you want to be able to ignore things).
If you do things the way your users are familiar with, they will be happier; conventional ways of using features have stood the test of time, but any innovation you make has not and thus may suffer from hard-to-anticipate problems.
Different users have different preferences for interaction styles, and some users have physical limitations that make it difficult or impossible to use certain features. A blind user, for example, can work with textual menus using a device that translates on-screen text to audible speech, but graphical icons can't be translated by the device. A person with impaired motor control may be able to type but may not have the fine hand control needed to use the mouse, while a person with the use of only one hand might have problems with two-key combinations and prefer the mouse and pulldown menus. For all of these users, providing more than one access to a function may be essential.
All of these statements are abstract. It can be hard to see how or whether they apply to a given design problem without some experience in applying them. Spend some time looking at a good interface and seeing if you can make sense of its features in terms of these ideas.
Back to Chapter 3
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