One of the most ignored principles in user interface design - yet one of the most obvious - is Fitts's Law:
The time to acquire a target is a function of the distance to the target and the size of the target
This has numerous implications for interface design. For example:
- Use large targets (e.g. big buttons) for functions users need to perform frequently.
- Use small targets for functions you want to make it harder for users to perform (e.g. delete).
- The edges of the screen are a great location for targets, as they are effectively infinite in one direction, but only if there is no padding between the target and the edge of the screen. For example, a palette of controls arranged in a single row along the edge of the screen will be faster to use than a double row of controls. Top of screen menus are easier to access than top of window menus.
- The corners of the screen are an even better location for targets, as they are effectively infinite in two directions (but remember that the user may have multiple monitors).
- The easiest pixel to click is the one that is always directly under the cursor - it's effectively an infinitely large target. Popup context menus make use of this.
- Incorporating labels on toolbar buttons makes them easier to click (as it makes them larger) and makes it less likely that you'll click the wrong one (as it spreads them further apart)
- Radial popup menus are faster to use than standard linear popup menus as every item is only one or two pixels from the position of the cursor. They can also utilise the user's motor memory.
Fitts's law is not just about the size of a target and the distance to it. It's also about the number of targets. For example, hierarchical menus are hard to use due to the difficulty moving between the first-level menu and the second-level menu. The first-level menu is an extra target that the user must hit before they can navigate to their desired target, the second-level menu, and only a narrow range of vertical movement is tolerated before the second-level menu will disappear.