Paul M. Fitts (1912-1965) was a psychologist at both Ohio State University and the University of Michigan. In 1954, he created a mathematical formula to determine the relationship how long it takes a user to either select an object on the screen, or by physically touching it, based on it's target size and distance from the selector's starting point. Fitts' Law is widely used today by UX designers, human factor specialists and engineers when designing graphical user interfaces and comparing performance of various input devices. Fitts Law finds that:

• The time required to move to a target is a function of the target size and distance to the target.
• The further a target object is from the initial starting position, will require a longer time to make that successful selection.
• That time can be increased when the target size is too small.

In mobile devices, we know that screen display size is limited and its space is valuable. In addition, mobile users require quick access to the content they are looking for. Using Fitts' Law together with these constraints, can improve the user experience.

• Buttons and selectable controls should be an appropriate size because it is relatively difficult to click on small ones. Using the screen bezel overflow can provide a trick allowing smaller, yet highly selectable. For mouse or other pointer-driven systems, the edge of a selectable area becomes "infinitely" deep, so the selectable area is functionally much larger. For touch devices with a flat bezel, the user may place part of their finger off the screen and activate a much smaller target than usual. Refer to the General Touch Interaction Guidelines in the Appendix.
• Accessing information using pop-ups and tooltips can usually be opened or activated faster than pull-down menus since the user avoids travel.
• Reduce the number of clicks to access content by providing surface level sorting and filtering controls to access indexed information quickly.