A theoretical model is developed that predicts upper-bound and lower-bound text-entry rates using a stylus to tap on a soft Qwerty keyboard. The model is based on the Hick-Hyman law for choice reaction time, Fitts' law for rapid aimed movements, and linguistic tables for the relative frequencies of letter-pairs (digrams) in common English. The model's importance lies not only in the predictions provided, but in its characterization of text-entry tasks using keyboards, and in the model's utility in optimising soft keyboards. An important contribution is the construction of a complete table of digram frequencies including the space character.; The accuracy of the model is evaluated in an experimental context.; A review of the interest in soft keyboard optimisation generated by the publication of the model is presented.; A second model is presented that predicts upper-bound text entry rates for two-thumb typing on a miniature handheld Qwerty keyboard. This model is of interest because it presents a deconstruction of the two-thumb text entry task, and because it makes a priori evaluation of miniature keyboards possible. Important contributions include the construction of two Fitts' law models describing the motion of the thumbs during two-thumb typing, and measurement and modelling of key repeat and thumb alternation times.
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