  Some individuals with motor impairments communicate using a single switch --
such as a button click, air puff, or blink. Row-column scanning provides a
method for choosing items arranged in a grid using a single switch. An
alternative, Nomon, allows potential selections to be arranged arbitrarily
rather than requiring a grid (as desired for gaming, drawing, etc.) -- and
provides an alternative probabilistic selection method. While past results
suggest that Nomon may be faster and easier to use than row-column scanning, no
work has yet quantified performance of the two methods over longer time periods
or in tasks beyond writing. In this paper, we also develop and validate a
webcam-based switch that allows a user without a motor impairment to
approximate the response times of a motor-impaired single switch user; although
the approximation is not a replacement for testing with single-switch users, it
allows us to better initialize, calibrate, and evaluate our method. Over 10
sessions with the webcam switch, we found users typed faster and more easily
with Nomon than with row-column scanning. The benefits of Nomon were even more
pronounced in a picture-selection task. Evaluation and feedback from a
motor-impaired switch user further supports the promise of Nomon.
