One question that often gets asked, both at my talks and on projects, is how many gestures can a user reasonably be asked to remember in order to control a system?
The honest answer is: I don’t know, and I doubt anyone else does either. I’d love to see a research paper on this. There are a number of points of data for it, however.
First is sign language. Signers have an extensive vocabulary of gestures in order to communicate. So clearly the upper limit of the number of gestures any human being can learn is at least in the hundreds and likely in the thousands if necessary. Of course, no user is likely to learn anywhere near that number to control a system. Signers do it out of necessity of communicating.
Looking at the world of sports, there seems in most physical games a core set of movements necessary to play most games. The number of movements for any given game is usually fairly small (although of course the mastery of these requires intense practice). From my observation and rough estimate, the core movements of most sports are less than seven, and that is counting actions like running and jumping.
Musicians likewise have a core set of gestures, and again, this number seems to be less than seven. If we take playing a stringed instrument, for instance, those are pressing and holding strings down, sliding the hand up and down, sliding the hand left and right, bowing or strumming, plucking or picking. It’s the combination and nuance of performance that brings the instrument to life and makes music.
When we look at the digital world, I’m sure somewhere there is an academic paper on the number of keystroke commands (probably the most cognitively similar thing to interactive gestures on traditional WIMP systems) that users remember. I’m guessing this too hovers around five to nine per application, with of course some key commands (control-s for save for instance) spanning across multiple applications. Certainly some advanced users (just like professional musicians and athletes have a more advanced set of actions) will know more key commands for specific applications that they spend a lot of time working with.
The iPhone’s set of gestures is nine: tap, double tap, flick, drag, swipe, pinch open, pinch close, touch and hold, and two-finger scroll.
So to sum up, we don’t really know yet, but the magical number would seem to once again be seven, plus or minus two. Unless you are planning on training users, having a core set of five to nine gestures that can be remembered and perhaps even used in combination seems to be a good practice.