[IxDA Discuss] Fitt's law for mobile devices

[Peter Bagnall]

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/