unity-design team mailing list archive

[Ed Lin <edlin280@xxxxxxxxx>]

I already said this but I'll repeat it to prevent being misunderstood:
I maintain that Fitts's Law is absolutely accurate and helpful for
what it describes. The problem is that it doesn't describe all factors
necessary for a full assessment and is often applied in a too simple
and linear (literally...) fashion.

On Mon, May 23, 2011 at 4:36 PM, David Regev <david.regev@xxxxxxxxx> wrote:
> W is the width of the target. Perhaps you’re thinking about D. As I
> explained, for any particular display, in practice, you can figure out a
> specific value for that based on how far users typically travel past the
> edge. That’s exactly what Jef Raskin did. If you think this is inaccurate,
> you need to do your own studies and measurements. (I wish someone would
> actually do that on Ubuntu.) In any case, the point stands: Fitts’ Law
> doesn't break down at screen edges.

Fitts's Law is based on the assumption of targets with fixed W.
Targets on the edge aren't fixed, their "size" (=how far the pointer
is moved further) depends on D. No testing needed for that, it's
obvious and doesn't depend on any specifics of the implementation.
To get more accurate results you'd need to determine several values
for W depending on D and angle.

> It does take this into account—a and b depends on he context, and must be
> determined through testing. I suspect that you’re right that the angle
> changes things (though very slightly). This is why Tog (question 3) was able
> to list the corners in order of difficulty to access.
>

It does not take into account strain or inertia. According to the
formula small nearby targets are as easy to hit as large targets
further away. But the basic laws of physics (and physiology) disagree.

The second aspect of my argument was that while the formula can be
adapted for 2D surfaces with different properties depending on angle
usually this is neglected.

> The average distance is not the same. That wouldn’t make any sense.
> Windows-style menus and Macintosh-style menus obviously have different
> distances. And that’s the point: you can compensate for a larger distance
> with a larger width, which the edge will give you (and it’s not ∞).

In "The humane interface: new directions for designing interactive
systems" Jef Raskin uses the same distance for "Mac" and "Windows"
menu, see the link you posted. It's so obviously flawed that I
question his objectivity. He "won", but on a little 14 inch and only
if single tasking and ignoring that the pointer needs to travel back
from the top to the window . A lot of ifs....

> Anyway, he gives you a and b and the distance and width for both styles of
> menu. (He uses different letters for the variables, though.) You can do the
> calculations yourself and verify. If you do, I hope you’ll post the results.
>
> Raskin was using whatever mouse acceleration existed on the Macintosh at the
> time of his test. I have no idea how accurate it still is for modern
> systems, but I am not aware of any more recent determinations of a and b for
> such systems. In any case, better mouse acceleration would actually make the
> screen edge even easier to reach.

a and b don't matter for our discussion as they are constant, so yeah,
my acceleration argument is probably a red herring :P

>> 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.
>
> And as all researchers already know, and as I’ve seen pointed out in most
> serious discussions of Fitts’ Law. The equation describes movement in a
> straight line. Further movement in another direction is a separate
> calculation, which is why the target’s horizontal width is also important,

You'll find that for the second calculation D of the first will be
very important. If we start comparing different interfaces with
milliseconds we can't brush that aside. Your two links seem to do just
that.

>> These are all things UI designers should keep in mind. Fitts's Law is only
>> one aspect and in many ways an oversimplification.
>
> Exactly. And that’s why you shouldn’t claim that Fitts’ Law doesn’t take
> things into account. It’s people who may neglect taking certain factors into
> account, like the time added up outside that one linear movement that the
> Fitts described.
>
I can agree with that. with a few buts as noted: field of vision,
visibility of the target (hover menu), wrist strain and all sorts of
side effects such as in the case of the global menubar the
multi-tasking or discoverability.