unity-design team mailing list archive

[Ed Lin <edlin280@xxxxxxxxx>]

On Sun, May 22, 2011 at 11:28 PM, David Regev <david.regev@xxxxxxxxx> wrote:
> 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, for example, found that W was 50
> mm for the Macintosh.

W in this context depends on acceleration curve and distance, it's not
fixed! That's the reason one may be tempted to start calculating with
infinity and mathematically speaking its use is absolutely correct.
"You are not supposed to" doesn't count.

> 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 [1] than your
> typical large monitor).
>
[1] http://ubuntuforums.org/showthread.php?p=3494680#post3494680

I doubt these results. The mouse doesn't accelerate perfectly and
indefinitely. There is a typical maximal speed people move their
hands. Ever noticed how moving the pointer in a diagonal (bottom left
<-> top right for right hander) or horizontal line is easier to do
fast than moving vertically? The menubar usually has to be accessed
with a vertical movement, squarely to the top left. _Again_ something
Fitts's law does not take into account. Apart from requiring more
"work" from the user to access targets which are further away, no
matter the size,  it also adds to the RSI risk.

Sadly he doesn't share his calculations with us but I also think his
math is flawed because it's based on Raskin's who uses the same
average distance both for the global and the in-window menus. It also
looks like he didn't correct for mouse acceleration.

Overshooting the top menu usually results in lateral movement which
can put the pointer above the wrong entry. The horizontal W becomes a
pretty small number. As mentioned the simple formula used ignores
that.

These are all things UI designers should keep in mind. Fitts's Law is
only one aspect and in many ways an oversimplification.