Interaction design laws
Fitts's law
Also called Fitts' law
The time to reach a target depends on how far away it is and how big it is: close, large targets are fast to hit, small distant ones slow.
Pointing is quick when the target is big and near, and slow and error-prone when it's small and far. On touchscreens, the target also has to be big enough for a fingertip.
Make frequent and important actions large and close to where people's hands or pointers already are, and keep destructive actions away from common ones.
In practice
Putting a phone app's main action at the bottom of the screen, within reach of a thumb, rather than in the top corner.
Common mistake
Tiny tap targets packed together, or a Delete button right beside Save.
Where it comes from
Described by psychologist Paul Fitts in 1954, from experiments in which people tapped between targets of different sizes and distances.