[IxDA Discuss] Fitt's law for mobile devices

[Xiaoyu Chen]

Hi Robert,

Thank you for the inspiring inputs to this topic. I just want to add two
cents and hopefully to give you a reference that time is not distance when
we talk about strict science. I am from the academia, but I will try to use
non-jargons to explain.

The way the laws on human computer interfaces were generated is strictly
controlled scientific experiments. For example, to repeat the results of
Fitt's law on a computer interface, you can set up an experiment where you
prepare an interface with image icons of different sizes and distances to
each other. You then ask a number of experiment participants to move the
mouse between different pairs of icons. You collect the times they use to
move between icons. You also have the information of the sizes of and the
distances between icons. You then can use statistical methods to test
whether your data produces a mathematical model that's the same as Fitt's
Law. If you are not familiar with Fitt's law's mathematical expression you
can refer to this link: http://en.wikipedia.org/wiki/Fitts'_law.

In all scientific experiments time and distance are two distinct measures
and can only be measured separately. If an experiment proofs a relationship
involving a physical distance, this relationship is only valid with the
distance and not time. If you need time to be encompassed, you need new
experiments to proof a new relationship.

What Fitt's law does is predicting the user's movement time between two
interface objects in *one operator* based on the objects' sizes and physical
distance. What you observed from your study encompasses *a series of
operators* where not Fitt's Law, but GOMS model can give a good explanation
(http://www.usabilityfirst.com/methods/goms.txl).

Hope this helps. Feel free to add in more thoughts.


Xiaoyu



On 8/15/06, Robert Hoekman, Jr. <rhoekmanjr at gmail.com> wrote:
>
> [Please voluntarily trim replies to include only relevant quoted
> material.]
>
> I can see what you're saying - really. I'm still not convinced I agree
> that
> the law can't be interpreted to encompass time as distance, but the
> separation isn't a bad idea either.
>
> The fact that you said I've found "Hoekman's Law", of course, makes me
> want
> to stop debating the point and just run with it. :) It seems way too
> obvious
> to call it a law, but there are loads of laws that exist only to remind us
> of the obvious. The fact, is, a lot of people need to have the obvious
> pointed out to them.
>
> -r-
>
> On 8/15/06, Peter Bagnall <pete at surfaceeffect.com> wrote:
> >
> > On 15 Aug 2006, at 16:45, Robert Hoekman, Jr. wrote:
> >
> > What we discovered, quite obviously, is that the further away the task's
> > completion point was from the task's starting point, the more difficult
> it
> > was to complete the task and the longer it took.
> >
> >
> > Indeed, that's is precisely what I would have expected too, but it's
> still
> > not Fitt's law. You can't use Fitt's law to predict this effect. What
> you've
> > found, by doing a small experiment is Hoekman's law ;-)
> >
> >
> > This has a direct correlation to Fitts' Law, even if the law wasn't
> meant
> > to measure time.
> >
> >
> > If you'll forgive me for being even more picky (wow, is that
> possible?!),
> > it's not a correlation, it's an analogy.
> >
> >
> > So, even if the original intent was to measure only physical distance,
> the
> > Law can be adapted to include time as well. It "fits" way too well to
> ignore
> > it. Time is distance too.
> >
> >
> > I have to disagree, the law cannot be adapted, that's exactly what I'm
> > objecting to. The law is derived from experimental data gained under
> precise
> > conditions. What you're suggesting is quite different from those
> conditions
> > and so the experiment that Fitt carried out says absolutely nothing
> about
> > it.
> >
> > I agree that since time and space are analogous it is reasonable to
> guess
> > that a similar law may also exist for time. But until you actually do
> that
> > experiment you don't know if that assumption is true. Science does not
> > proceed by assumption. Since you have done an experiment though, we now
> know
> > that such a law does exist, but law is *still* not Fitt's law. It's a
> whole
> > new law (actually, I'd call it a model, but I think I've probably split
> > enough hairs for one day).
> >
> > If you'd not done the experiment, for our purposes the assumption would
> > probably be safe, since it's pretty reasonable, but it would be much
> more
> > accurate to say "by analogy to Fitt's law we expect...." instead of
> saying
> > "Fitt's law says...".
> >
> >
> > May be a case of "the spirit of the law" winning out over "the letter of
> > the law", but it's hard to deny the connection.
> >
> >
> > It's very tempting to do this, but as soon as you do you invalidate the
> > science. It's fine to look for connections like that, but not everything
> > that's obvious is true, that's why the scientific method exists.
> >
> > I'm not trying to say that you shouldn't reason by analogy - all I'm
> > saying is that when you cite a scientific result be very careful that
> you
> > pay close attention to the limits of the experiment, and don't use
> results
> > beyond the limits of their validity. So if you're using the "spirit" of
> the
> > law, then you must say so, rather than claim to be using the "letter" of
> the
> > law - that's all I'm really trying to get at at the end of the day.
> >
> > Cheers
> > --Pete
> >
> > ----------------------------------------------------------
> > Always design a thing by considering it in its next larger
> > context - a chair in a room, a room in a house, a house in an
> > environment, an environment in a city plan.
> >      - Eliel Saarinen, 1873 - 1950
> >
> > Peter Bagnall - http://people.surfaceeffect.com/pete/
> >
> >
> >
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