Page 154 - Handbook of Electronic Assistive Technology

142 HANDBOOK OF ELECTRONIC ASSISTIVE TECHNOLOGY

Error Handling

Scanning with switches is a slow method of access and errors of selection have considerable
detrimental impact on efficiency because of the time required to delete the erroneous selec-
tion and then generate the correct command. It is important to design selection sets and con-
figure the system to minimise errors, and to provide mechanisms for the user to delete or
cancel incorrect selections.

Switch Settings

Scan Time The time for the scan to move from one item to the next.
Initial Scan Time The time that the first item is highlighted – this gives the user additional time to select the
first item.
Dwell time The length of time that the item is highlighted after which it will be selected automatically.
Dwell selection is used instead of a separate switch.
Debounce time A time delay that ignores additional switch activations following the initial activation. This
filters out mechanical switch bounce or user tremor.
Acceptance time The time that the switch must be held down before it is accepted by the system. This can
filter out accidental unwanted switch activations.

Rate Enhancement and Speed of Access
Switch and scan access is slow: Koester and Arthanat (2017) report text entry rates averag-
ing 1.67 WPM across 14 studies and 34 subjects for scanning users, compared with 15.4
WPM for speech recognition; 12.5 WPM for typing with a standard keyboard; and 4.2 WPM
for mouse access to an on-screen keyboard. Therefore it is essential that switch and scan
systems minimise errors and provide mechanisms to increase speed of access, reduce
switch activations and physical effort and improve ease of use.
Speed of access can be increased by considering aspects of the switch and scan
mechanism:
• Identifying the most suitable switch and switch control site.
• Selection set design – for example, by only offering items within the selection set that
are required for the task in hand, or by adopting a frequency of use layout, where the
most common items or keys can be accessed more rapidly.
• Careful adjustment of settings such as Scan Time – clearly, a faster Scan Time will enable
faster access, provided that the user can select items accurately.

In addition, there are other techniques that can be explored to increase rate of access.
Word prediction reduces the number of selections required to generate text
(Higginbotham, 1992) and therefore we might assume that it will increase speed of text
prediction when scanning, but this is not strongly supported by the few studies that have
been published (Koester and Levine, 1994; Pouplin et al., 2014). The fact that research does
not support the use of the technique does not seem to discourage many switch users (e.g.,
Professor Stephen Hawking) who used word prediction.