The equation doesn’t break down here because you’re not supposed to put in ∞. Although the edge may theoretically be infinitely long, it’s not in practice. Any human being aiming at the edge will stop within a certain distance past the edge. That means that, effectively, *W* is a certain fixed quantity, which can be determined through testing and seeing how far users actually travel past the edge. Jef Raskin<http://books.google.com/books?id=D39vjmLfO3kC&pg=PA94>, for example, found that *W* was 50 mm for the Macintosh. Using these methods, Fitts’ Law will get you more accurate results. You can also figure out how great *D* must be for global menus not to be faster any more (hint: probably much larger<http://ubuntuforums.org/showthread.php?p=3494680#post3494680>than your typical large monitor).
On Sun, May 22, 2011 at 16:53, Ed Lin <edlin...@gmail.com> wrote: > I'd also argue that while mathematically sound (at first glance) literally > putting in infinity doesn't give you a realistic result, think divided by > zero which can break an otherwise perfectly usable formula. Put in a > ridiculously large "D" and with infinitive "W" you'd get exactly the same > result (you'd calculate with zero) which doesn't take in account b, the > inherent speed of the pointer. Clearly the law fails here. Just see the > derivation: natural number =∞/2 ?? Someone with a bit more math background, > please chime in. >
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