- Fitt’s Law declares that performance is logarithmic. It utilizes an UI element’s ratio of distance (to it) to (its) width.
- A + B * log2(D/W)
- log2(D/W) is the index of difficulty in bits.
- 1/B is the index of performance in bits per second.
- Fitt’s Law can be used to calculate the difference in index of difficulty between Mac and PC menu designs. When using indirect pointing (a mouse), the barrier behind the Mac’s menu button turns its target size from 20 to infinity — the W used in the equation is the button’s width, 60. The Mac’s index of difficulty is log(600/60) = log(10), compared to PC’s log(20). The Mac menu button has a lower index of difficulty.

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