Analysis of timings on real-world interfaces sometimes yields
suggestive curves. That is, suggestive to a statistician, who may be
able to recognize a powerful qualitative principle in the numbers.
One such regularity is *Hick's
Law*,^{[26]} which for our purposes we
can paraphrase as: *The time M(n) required to make a choice
from a menu of n items rises with the log to the base two of
n*.

The key fact here is that the rise of M(n)
is sublinear. Thus, the Rule of Large Menus:
*one large menu is more time-efficient than several small
submenus supporting the same choices*, even if we ignore the
time overhead of moving among submenus.

A related result is *Fitts's Law*, which
predicts that the time T(D,S) to move the mouse pointer to a region on a
screen, when is initially D units away from the region border and the
region is S units deep in the direction of motion, rises with the
log to the base two of D/S.

Since you can't control D — the user
will start his mouse movements from unpredictable locations — it
follows that the way to reduce the D/S ratio that predicts select time
in a point-and-click interface is to *make the target
larger*. But not huge; the log to the base two in the
formula means that, as with Hick's Law, the efficiency gains are
sublinear and fall off as the ratio rises, and that gain has to be
traded off against the value of other uses for the screen
space.

Still, we get from this the Rule of Target Size: The size of a button should be proportional to its expected frequency of use.

Macintosh fans like to point out that Fitts's Law implies a very
large advantage for Mac-style edge-of-screen menus with no borders,
because they effectively extend the depth of the target area
offscreen. This prediction is borne out by experiment. Under Unix. we
can capture this benefit with a (borderless) taskbar adjacent to any
screen edge, but the standard Unix toolkits don't give us a way to
harness it within applications (because application windows have
borders and anyway don't typically appear nestled up to a sctreen
edge).^{[27]}

We can get from this the Rule of the Infinite Edge: The easiest target rectangles on the screen are those adjacent to its edges.

Hick's Law and Fitts's Law come from a place even deeper than evolved human instinctual wiring. They're related to the Shannon-Hartley Theorem in information theory and would probably just hold as true for intelligent squids, robots, or anything else with an eye-brain-hand loop that has to check whether the mouse pointer has landed in the right spot by tracking progress against a visual boundary.

For more on the application of Fitts's Law, including some
marvelously detailed case studies, see Bruce Tognazzini's February
1999 *Ask Tog*^{[28]}.

^{[26] }For the precise mathematical
statements of both Hick's Law and Fitts's Law, see the discussion in
[Raskin].

^{[27] }In fact Fitts's Law tells us that the most
easily targeted areas would be the (unbordered) four corners of the
screen, which have offscreen landing zones on two sides. No GUI
toolkit in history has actually used this
fact.