A Probabilistic Character Layout Strategy for Mobile Text Entry

We present a text entry strategy designed for small, input-limited devices in mobile situations. In an existing text entry interaction style, arrow keys move a cursor around the character set, displayed on a 3 or 4 line liquid crystal display, and a select key outputs the highlighted character to the display. The Fluctuating Optimal Character Layout (FOCL) strategy aims to improve entry rates with this interaction technique. After each character c entered, the layout is rearranged so that the most likely next characters are closer to the cursor. Each new layout is optimal with respect to c, given digram (letter-pair) probabilities in common English. FOCL significantly reduces kspc-the average number of keystrokes per character-, a non-empirical measure that partly accounts for text entry speed. However, it also requires the user to locate each character in an unfamiliar layout, thus adding visual search time to the task. In a longitudinal experiment comparing the fluctuating layout approach to a fixed QWERTY layout, we found no significant difference in entry speeds. We discuss our design rationale and various modifications to the design that may yield a performance improvement.