Fitts' Law: The time to acquire a target is a function of the distance to and size of the target.
Index of difficulty = log2(2d/w)
where d is the distance to target and w the width of the target
Time to point = c1 + c2 * (Index of difficulty) where c1 and c2 are device dependant constants.
[The mouse] is a “Fitts’s Law” device. That is, after some practice you can point with a mouse as quickly and easily as you can with the tip of your finger. The limitations on pointing speed are those inherent in the human nervous system.
time for reaching the target for a single-level pull-down = 1.08 + 0.096 log2(d/w +0.5) sec
where w is the height of the menu item
Doug Engelbart and Bill English concider the mouse as a interim device. Bill English tried to develop a device that is faster than the mouse, but the mathmatical equation of the mouse is pretty much the same than a plain hand pointing action. Therefor the speed limiting factor is not the mouse device but the eye-hand coordintae system itself.
“Furthermore, the model provided guidance for interface designers: make distant buttons large, for example. These studies were heavily used in the debate within Xerox that lead to the decision to depart from tradition by including a mouse with the new Star product.”
The border of the screen is infinite.
Mouse movements constraints to be added to border of popup menus, to top of windows if it containes a menu bar like Windows™ can speed up menu access.
Fitts’ Law applied for high-precision touchscreens. Added elements to the definition to reflect fine-tuning motions of fingers [Shneiderman 98, p. 325]
describes lucidly what Fitts’ Law is, how it has been applied, and what the many refinements are, i.e. two dimensional pointing
For a two-dimensional target, you can usually obtain a reasonable approximation of the time needed to move the cursor to the target, using the samaller of the horizontal and vertical dimensions of the target for the value w. [Raskin 2000, p.94]
Mac menu vs. Windows menu.
Acquisition of Expanding Targets. CHI 2002
Take a look at Michael McGuffin's master's thesis Fitts' Law and Expanding Targets: An Experimental Study, and Applications to User Interface Design