Ease of juggling: studying the effects of manual multitasking

Everyday activities often involve using an interactive device while one is handling various other physical objects (wallets, bags, doors, pens, mugs, etc.). This paper presents the Manual Multitasking Test, a test with 12 conditions emulating manual demands of everyday multitasking situations. It allows experimenters to expose the effects of design on "manual flexibility": users' ability to reconfigure the sensorimotor control of arms, hands, and fingers in order to regain the high performance levels they experience when using the device on its own. The test was deployed for pointing devices on laptops and Qwerty keyboards of mobile devices. In these studies, we identified facilitative design features whose absence explains, for example, why the mouse and stylus function poorly in multi-object performance. The issue deserves more attention, because interfaces that are nominally similar (e.g., "one-handed input") can vary dramatically in terms of "ease of juggling".