Keyboard 'bypass' techniques allow physically disabled people, otherwise unable to use conventional means of text composition, to do so. Ability to compose text is extremely important to the disabled, offering potential for non-speech communication, computer access, creative writing, etc. Consequently, these techniques have received a good deal of attention and many diverse systems have evolved. They all suffer, however, from the drawback of inherently slow input, quite apart from any disability on the part of the user. For this reason, text composition rate (or communication rate) is the major figure of merit. Since many diverse systems and approaches exist, quantitative methods of comparison are required to guide prescription and development of such aids. Only recently have attempts to produce models which predict communication rate been made. This paper extends the earlier model of Rosen and Gooenough-Trepagnier to encompass scanning-input systems. Scanning input is of considerable interest since it can be used by very severely disabled people. The model developed is applied to the comparison of two very different systems: row-column scanning and the 'scanning Microwriter'. According to the model, row-scanning is very much faster than the scanning Microwriter when a letter-frequency arrangement of the character selections is used. The relation of the model to classical information theory, treating the disabled user as an information source, is also explored.