People with severe motor disabilities have extreme access difficulties with all kinds of web services especially when they want not only to surf the web, but also write some text, e.g., to participate in an e-activity. Several problems arise when using traditional scanning systems, such as the low text entry rate, the time consuming task of learning the scan matrix layout, or simply, the poor visibility of the web page due to the large surface needed to display the complete scan matrix on the screen. We propose a reduced virtual keyboard based on scanning with only one switch as input device. The scan matrix consists of only three cells, so ambiguity is present due to the assignment of 26 characters to the three keys. Word-level and character-level disambiguation modes are explored using a mathematical model, and the text entry rates for an expert user were 15.9 and 10.3 words per minute respectively, using a scan period of 0.5 seconds. This keyboard could be embedded into a web page using a Java applet, JavaScript code or a Flash application, or be programmed as an independent application.

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This paper focuses on the relevance of the cell selection phase in the overall performance of a text entry system based on scanning and with an ambiguous keyboard. Most of the layouts are designed trying only to minimize the ambiguity of the keyboard, and taking into consideration only the disambiguation process when entering text. Nevertheless, the number of scan cycles necessary for selecting the cells has great importance in the overall performance. As we show, the performance depends on the number of cells and the linguistic model used in the cell selection phase.

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