Anthropometrics: Important Aspects of Users’ Bodies - Foundations for Designing User-Centered Systems - page 63

Fig. 3.7 A touch screen used
to provide voting without a
paper trail (left), and a GPS
navigation system (right)

Even when touch screen technology is both usable and acceptable to the users,
there can be other risks that mitigate against its use in some situations. The touch
screen voting system shown on the left side of Fig. 3.7 , for example, was removed
within a year of being introduced because it could not produce a voter-verifiable
paper audit trail. In other words, there was no way for a voter to tell that their vote
had been properly recorded rather than discarded after they had pressed the
appropriate buttons. The public and public officials believed that the risk of
potential corruption—the voter gets feedback from the device saying the vote has
been cast, but the vote never gets processed to include it in the count—made the
system unacceptable.

3.3.3 Pointing Devices

People still widely use mice as a way of interacting with systems and applications,
although usage may decline as the uptake of touch screen technology continues to
rise. Although the mouse is still the most common form of pointing device, there
are others, such as trackpads, graphic tablets, tracker balls, and light pens.
Recently released devices, such as Google Glass, are increasingly exploring the
pros and cons of gaze as a means of pointing. However, for the time being, the
dominant mode of interaction with desktop computers is still the mouse.
The movement of on-screen cursors using pointing devices generally follows
Fitts' ( 1954 ) law. In its most general form, Fitts' law states that the time to point to
an object is related to the distance from the object and inversely related to the size
of the object, as shown in Fig. 3.8 .
There are several variants of Fitts' law. The simplest is shown in Eq. ( 3.1 ):

time ¼ intercept constant þ slope constant log 2 ð 2 d = w Þ ð 3 : 1 Þ

Card et al. ( 1983) variant shown in Eq. ( 3.2 ) is slightly easier to use:

time ¼ 70 ms log 2 d = w þ 0 : 5 ð Þ ð 3 : 2 Þ