Fitts' Law offers some theoretical predictions on how quickly an individual can make choices of targets of a given size located a given distance from a starting point. Since the application of Fitts' Law in the clinical setting would require information not generally available, the more practical approach is actual trials on keyboards of different sizes, keeping these principles in mind. The greater the number of keys, the higher the information rate. The larger the keys, the easier they are to activate. The most effective users of AAC access core and extended vocabulary using different methods.
Core vocabulary constitutes the vast majority of communication.
Therefore, consideration of these issues is most important as applied to core vocabulary access. A chart is provided here to facilitate this discussion. The first of these issues is the size of the symbol set used in the vocabulary representation method. For maximum performance symbol set size should not exceed the number of locations in the selection array. When this happens, the selection array must be changed during use of the system to accommodate the required number of symbols.