  Patients with motor control difficulties often "type" on a computer using a
switch keyboard to guide a scanning cursor to text elements. We show how to
optimize some parts of the design of switch keyboards by casting the design
problem as mixed integer programming. A new algorithm to find an optimized
design solution is approximately 3600 times faster than a previous algorithm,
which was also susceptible to finding a non-optimal solution. The optimization
requires a model of the probability of an entry error, and we show how to build
such a model from experimental data. Example optimized keyboards are
demonstrated.
