Fitts's Law for Asymmetric Targets (Like Tabs)

Mon 29 June 2009
By Martin Polley

Since I started putting together my entry for the Mozilla Labs “Reinventing Tabs” challenge, I have been thinking about Fitts’s Law. Here’s what I have been thinking: does the shape of the click target affect the ease of acquisition?

More simply put, is it easier or more difficult to acquire a rectangular target that is (say) three times as wide as it is high than a square target of the same area?

And also, does the orientation of the rectangle make it easier or harder to acquire? (That is, does it make any difference whether the rectangle’s long side or short side is facing towards the pointer?)

Why does this matter? Well, if you have a bunch of tabs stacked up the left side of the screen (as in my entry), the answer could mean that they are easier or harder to click on than regular tabs of the same size that are strung out horizontally above the content area.

And now I have found a paper that contains the answer. At least, it contains the answer to the second part of my question (about orientation).

If you want to read it, go ahead, but here is the money quote:

... when possible graphical widgets should be extended along
the more frequent movement direction. ... horizontally enlongated widgets, which are often due to the labeling in English words, should be placed on the left or right rather than the top or bottom edge of the desktop interface.

But there is a caveat:

However, the average horizontal movement distance is somewhat longer due to the landscape display geometry in most computers.

In plain English, tabs that are stacked at the side are easier to acquire than ones of the same size and shape along the top. But because monitors (especially modern wide-screen ones) are wider than they are high, this advantage may be mitigated (or even outweighed) by the extra (average) distance that the pointer must cover.