TBS3: Two-Bar Single-Switch Scanning for Target Selection

We present a two-dimensional (2D) pointing technique for motor-impaired users who generally use single-input switch. The technique, called TBS3 for "two-bar single-switch scanning", uses two bars and proceeds in two steps: moving a vertical bar then moving a horizontal bar. The user starts and stops each bar by activating the single input switch. Selection is made at the intersection of the two bars when the horizontal bar is stopped by the user. We present two variations of the technique. In the first ("one-way"), the bars move only in one direction, and return to their origin after each selection. In the second ("button"), the user controls the direction by a selecting an on-screen soft button before bar movements starts. The techniques were evaluated in a experiment with twelve participants using the 2D target-selection task described in ISO 9241-411. Due to the two-stage pointing process, the task was inherently slow with a mean movement time of 8.4 s per trial. The mean error rate was just under 10% with the one-way method (8.0%) more accurate than the button method (11.1%). Throughput was 0.31 bps overall with values of 0.26 bps for the one-way method and 0.36 bps for the button method.