Anthropometrics: Important Aspects of Users’ Bodies - Foundations for Designing User-Centered Systems - page 64

Fig. 3.8 Description of the
variables in Fitts' Law. X is
where the user starts to point
from, d is the distance to the
target, and w is the width of
the target
The argument to the log 2 function in these equations is a measure of the difficulty
of the move, measured in bits of information (Shannon 1948 ). It is sometimes called
the index of difficulty. As shown in Fig. 3.8 , the target distance, d, is measured from
the point where the movement starts, X, and the target size, w, is the width of the
target. If the movement was from above or below the target, the target size would be
based on the height of the target (MacKenzie and Buxton 1992 ). The intercept
constant and slope constant vary across tasks and across input modalities. Typical
values for the intercept constant are about 100 ms. The slope constant varies from
about 20 ms for very good input devices like using fingers directly, to 105 ms for
using arms directly as well as for mice.
Fitts' law is less accurate where the size of the object, or the distance that has to
be moved, are very large, for example, where the user's mouse reaches the edge of
the desk before the mouse pointer has reached the edge of the screen, so the user
has to pick up the mouse and move it back across the desk before the pointer can
be moved any further. Some versions predict a negative reaction time for large or
close objects (where d/w is 0, for example), and time to move to distant objects
tends to be underestimated. Also note that the units are arbitrary, as it is the ratio of
the target size to target distance that is used. Fitts' law is, however, a fairly robust
law, and has been usefully applied to many interfaces because it makes good
suggestions for interface design.
There are at least two implications for designing user interfaces that arise out of
Fitts' Law. The first is that larger objects lead to faster pointing times than smaller
objects. The second is that shorter distances also lead to faster reaction times.
Indeed, the fastest time is to move a very small distance towards an infinitely large
target. Moving the cursor to the screen's edge (with effectively an infinite target
size) is much faster than moving the cursor to a bounded (finite) box that is more
centrally located. So, menu bars at the very top of the screen are much faster to
access than menu bars that are offset from the edge. These implications have to be
traded off against other factors though, such as the size of the other objects in a
display, the size of the display itself, sequencing of tasks, and how often the
objects and commands are used.