Fitt’s Law

March 4, 2013

As all my students know, I’m a big fan of Fitt’s law. Again, Wikipedia has a really nice discussion of it (my first encounter came long before Wikipedia existed, via Bruce Tognazzini; alas I’m not immediately finding the article which I read back in the day, but he’s obsessed with it), and  Kevin Hale’s is also excellent.

Very rougly, Fitt’s law states that the time to acquire a target (by pointing) on a linear path is proportional to the distance and the “width” (along the path) of the target. Big close things are easy; far small things hard hard to hit.

Well, d’uh-uhhhhhhhh. Right? Why is this even interesting?

Well, the qualitative statement doesn’t capture the actual, empirically backed, precise formulation. That alone is worth the price of admission. Getting precision out of obviousness is not easy, but doing so is worth it. Knowledge of the law drives a lot of the way interfaces work (e.g., the Mac’s fixed menu bar was explicitly fixed at the top to make it have effective infinite width).

It’s very universal, too. It applies across a wide range of populations and pointing modalities (cf. Hale for more discussion).

Fleshing it out to 2 (and 3) dimensions is also cool work. Exactly the kind of elaboration one would expect from a good basic piece of work.

Hale links to Yet Another Great Article which explores some consequences of the actual details of Fitt’s law.

So, minimally, I’d expect any computer science to at least recognize the name (“Fitt’s law”) and be able to recall the qualitative formulation. It’s a wonderful jewel of computer science and if you understand nothing else about HCI at least you’d have something profound, pervasive, and beautiful under your belt.

Even if you aren’t going to use it in your own work, it makes an excellent case to study for grasping how to go from an “obvious truth” to an empirically verified law, and why we want the latter. It’s really easy to design and even conduct simple experiments around the phenomenon, and it provides an analytical framework for thinking about a wide variety of issues in interface design. So, it’s a great practice area for people looking to study people.

Read up on Fitt’s law and try Tog’s quiz. It’s a great thing to think through.

Here’s a question of my own: Present a scenario where the acquisition advantage of the Mac Menubar is trumped by other factors. That is, when can having the menubar attached to a window (and thus being of finite length) win and win hard? (Use Fitt’s law!)

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