Fitts at 50: For Link Design, Size Does Matter

I missed the 50th anniversary of Paul Fitts’ paper "The information capacity of the human motor system in controlling the amplitude of movements". I wasn’t exactly expecting street parties, but June 2004 slipped by without even a celebratory cup of tea.

The title of the paper may not mean much on its own, but I end up discussing Fitts’ law with interaction designers at least once a week, so I would count it as one of the more robust works in our field. The conversations usually go something like this:

You have a news item on this page with a 2” square photo next to it, but the link is just this tiny icon at the end.

What do you mean?

The photo would make a better mouse target than the tiny icon. It’s called Fitts’ Law. Bigger targets are easier to hit.

He got a law named after him for that?

Well, there is a little more to it, but it’s an important principle in interaction design.

Surely nobody worries about how big the mouse targets are?

Not consciously, but it can slow people down, lead to errors and make life frustrating if you’re using a laptop on a train or don’t have high levels of manual dexterity.

But won’t a photo affect accessibility?

If it has meaning, you need to have descriptive (alt) text for it anyway. If it’s for decoration, you use a null alt tag so that assistive technologies such as screen readers don’t bother users with it. But you do that whether it’s a link or not.

Fitts very accurately related the difficulty of tapping a target to its size and the distance travelled. However, the size issue is not as straightforward as you might think. We are concerned with the size of the target in the direction of travel only. This is not much of an issue for most photos and images as these tend to be square, but text is a different matter. Consider the example in Figure 1.

Figure 1: Comparative target sizes for photo and line of text

If you are approaching either the photo or text target horizontally, their sizes are the same. However, a vertical approach is a different matter, with the photo being an order of magnitude larger in that axis. (Fitts law is a logarithmic relationship, so the photo will not be 10 times easier to hit than the text, but there will be a very noticeable difference.) While Fitts originally considered only one-dimensional horizontal or vertical movement, the principle has been extended to two dimensions through the simple expedient of taking an object’s size along the direction of travel (see MacKenzie and Buxton, 1992).

But that’s not all. There is ample evidence (Paivio, 1974, for example) that for many searching tasks, images are both quicker to find and more memorable than words. So not only are images easier to hit, but they are easier to find in the first place. Naturally there is a trade-off in terms of the amount of screen space occupied and the potential need for more scrolling, but where space allows, images have an important functional role to play. Even if images are not practical, raise a cup to Mr Fitts and make the text links larger.

Tips for Fitts

• Make images links, especially if they are already present.
• Make text links larger by using more words (within reason) or increasing the font size.
• Following Fitts’ Law, text links will be easier to hit if approached horizontally. Arrange your navigation so that the most frequent mouse movements are horizontal.
• Consider using icons (with appropriate descriptive text for accessibility) as well as text in order to increase target size, especially on pages where vertical mouse movement is expected.
• Small targets can be used to benefit interaction too. Make unusual or irreversible actions more difficult to select by reducing their size (relative to common actions).

References

Fitts, P M, 1954. The information capacity of the human motor system in controlling the amplitude of movements. Journal of Experimental Psychology, 47:381-391.

MacKenzie, I Scott and Buxton, William, 1992. "Extending Fitts’ Law to Two-Dimensional Tasks". CHI ’92, ACM.

Paivio, Allan, 1974. "Pictures and words in visual search". Memory & Cognition. Vol 2, 3:515-521.

The Author

William Hudson is principal consultant for Syntagm Ltd, based near Oxford in the UK. His experience ranges from firmware to desktop applications, but he started by writing interactive software in the early 1970's. For the past ten years his focus has been user interface design, object-oriented design and HCI.

Other free articles on user-centred design: www.syntagm.co.uk/design/articles.htm

© 2001-2005 ACM. This is the author's version of the work. It is posted here by permission of ACM for your personal use. Not for redistribution. The definitive version was published in interactions, {Volume 12, Issue 3, May + June 2005} http://doi.acm.org/10.1145/1060189.1060231