Fitts Law, applying it to touch screens

Question

Been reading a lot into UI design lately and Fitt's Law keeps popping up.

Now from what I gather its basically the larger an item is, and the closer it is to your cursor, the easier it is to click on.

So what about touch screen devices where the input comes from multiple touches or just single touches.

What are the fundamentals to take into account considering this?

Should it be something like, the hands of the user are on the sides of the device so the buttons should be close to the left and right hand sides of the device?

Thanks

Answer 1

I started thinking about this recently too, and here some considerations:

• Fitts' law was developed in the 50's as a human factors model (read: controls for fighter plane cockpits) so seeing it re-applied to human motor skills is actually just coming full circle. It definitely applies to mobile devices. [Historical note: The finding that it applied to mouse interfaces was actually a big deal at the time.]

• One thing to note is that the Fitts'-endowed advantages of the edges and especially corners of the screen no longer exist on a touch interface: the "infinite size" only applies to mousing interfaces since the cursor cannot move past the edges. Obviously, the same limitation does not exist for our fingers. Basically, the edges are no better than the middle of the screen except for the potentially shorter distance to the target.

• Here 1 (pdf) is an '06 study about optimal target sizes for one-handed thumb use, taking into account freedom of movement and such. I was hoping to find a paper that would be able to provide a modification or a new constant to Fitts' law for accuracy of the touch interface but a cursory search didn't turn one up. I guess that means I found a potential research topic ;)

• I think one general conclusion to be made based on application to Fitts' law to smaller-screened mobiles is that it's hard to make usable widget-based interfaces without seriously sacrificing information density. One interesting alternative is gesture-based interfaces (beyond the popular pinch and zoom). Unfortunately, the lack of popularity and conventions makes the learning curve rather high. Mobiles are definitely one place that it might be worth the trade-off, though. I predict wider adoption of gesture interfaces on mobiles once conventions stabilize.

Answer 2

Yes, for a touch screen Fitts' law has to be applied in three dimensions, so it's different from the classical mouse movement considerations.

As you say, the origin of the movement is often the default position of the finger. This varies a lot depending on the device where the screen is mounted. On a hand held device you might use the index finger of one hand, or the thumbs of both hands, depending on the design.

Also, on a touch screen you have to move the fingers away from the screen to see it, which makes the distance between controls less important as you move back to the default position between clicks.

What to consider besides Fitts' law is the intuitiveness of the interface. If a button appears where it's not expected, it doesn't matter how close it is, it will still take time to find it.