Matrices of the frequency and similarity of Arabic letters and allographs.

Indicators of letter frequency and similarity have long been available for Indo-European languages. They have not only been pivotal in controlling the design of experimental psycholinguistic studies seeking to determine the factors that underlie reading ability and literacy acquisition, but have also been useful for studies examining the more general aspects of human cognition. Despite their importance, however, such indicators are still not available for Modern Standard Arabic (MSA), a language that, by virtue of its orthographic system, presents an invaluable environment for the experimental investigation of visual word processing. This paper presents for the first time the frequencies of Arabic letters and their allographs based on a 40-million-word corpus, along with their similarity/confusability indicators in three domains: (1) the visual domain, based on human ratings; (2) the auditory domain, based on an analysis of the phonetic features of letter sounds; and (3) the motoric domain, based on an analysis of the stroke features used to write letters and their allographs. Taken together, the frequency and similarity of Arabic letters and their allographs in the visual and motoric domains, as well as the similarities among the letter sounds, will be useful for researchers interested in the processes underpinning orthographic processing, visual word recognition, reading, and literacy acquisition.