Statistical disambiguation of multi-character keys applied to reduce motor requirements for augmentative and alternative communication

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Abstract
The selection method employed in a communication aid is determined primarily by the ability of the user to interact accurately and rapidly with the device. The size of the targets (input switch, keys, symbols, etc.) and the number of targets combine to affect the overall success of the user. In the case of an orthographically based communication aid, a minimum of 27 keys or targets must be employed to allow unique direct selection of the letters of the alphabet and space. This approach is possibly compromised in the instance where a user's physical ability does not allow him/her to resolve the individual targets or extend over the range of targets. The effect of this is to introduce additional input errors, and/or reduce the input rate, and can result in the use of slower augmentative and alternative communication (AAC) methods such as encoding and scanning. This paper presents a new model for a communication system that reduces the number of keys to represent the alphabet from 27 to 11. The proposed keyboard employs nine keys, each representing a cluster of three letters, a tenth key dedicated to the space, and an eleventh used for error correction. The model uses fourth-order transitional probabilities to disambiguate among the characters of the three letter clusters. The model has been evaluated with several arrangements of letters against several texts. Measures are presented for the performance of the model in terms of the average keystrokes per letter and percentage correct disambiguation.

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