Fitt's Law applied to mobile devices has different implications for mobile UI design than it does for desktop design. Let's focus in on the dominant user interface type, the scroll-and-select device. Let's further assume a one-handed device - no QWERTY keyboards.

Hardware Design: What is close and what is far?

The device rests in the user's hand. The thumb can readily reach the number buttons on the phone, although some clamshell designs have a heavy top half, shifting the neutral resting point of the thumb. If we assume the user is using some type of application, one or both thumbs are in ready reach of the navigation buttons.

There are, on most phones, two key regions of buttons: numbers, and navigation (some or many of: softkeys, left/right/up/down, select, back, camera, etc.). Applications with intense number button use will find users with both thumbs hovering over the number buttons. Applications with intense navigation button use will find users with thumbs hovering over navigation buttons.

A mixed mode - number and navigation button use - is fine as long as neither is intense. Use slowed if the user has to do significant movement between regions due to shifting intensities.

Software Design: Optimizing for Speed

On a scroll-and-select device, distance is measured by the number of clicks to accomplish the task. Clicks include scrolling through the screen, pushing a Select button, pushing a softkey, scrolling through a softkey menu, and activating an item in a softkey menu.

Distances are increased when the hardware buttons are far from the user's current thumb position.

As an example, consider a Nokia phone, without a select button, on a standard Options/Back screen. Item X in the menu is highlighted. What actions are closest?

At 1 click, we can go Back (or Cancel, or whatever the right softkey is doing right now) --- or activate any control with a number shortcut.
At 2 clicks, we can select the currently highlighted item (Options -> choose).
At 3 clicks, we can select a neighboring screen menu item or a neighboring Options menu item.
And so forth.

On a phone with completely customizable softkeys, a Back button, and a Select button, what actions are closest? Let's assume that there are several commands available, and all but the most frequent are relegated to a Menu on the right softkey.

At 1 click, we can go Back, select the currently highlighted item, or the most frequently used command (on the left softkey) --- or activate any control with a number shortcut.
At 2 clicks, we can select a neighboring screen menu item or perform the second most frequent command.
And so forth.

So on a phone with unassigned softkeys, everything in the user interface is "closer" than it is for that Nokia device. (at the cost of more hardware buttons). The softkeys serve the same function as right-click on a Windows computer.

We now have a decent measure of what is "close", which can be performed on any device with any platform. Be sure to include platform considerations: while traversing 45 links on some browsers is 45 clicks, on the Opera Mini, with its left/right arrow page scrolling, it may be only 6 clicks.

So if you are designing for speed of interaction, this analysis suggests the following:

• When designing for Nokia devices, order the commands in the menu by decreasing frequency


• Provide numbered access for visible frequent items on the screen (as well as anything the user may have learned)


• Use the Select (OK) button, if available, to select and activate screen controls.


• On non-Nokia UI devices, assign the most frequently used command to the primary (usually left) softkey


• On non-Nokia UI devices, assign the other command, or a Menu label, to the secondary softkey.
  
• If a third softkey exists, assign a command to it.


• If creating a menu of commands, order them in decreasing frequency order.


• Allow lists, particularly command menus, to wrap so the user can push up from the top item to get to the bottom item.

Note: I have simplified phones into Nokia UI and non-Nokia UI for discussion purposes. More complexities exist.

Of course Fitt's Law is not the only consideration in user interface design - but it is an important one.