Adult developmental dyslexia in a shallow orthography: Are there subgroups?

The existence and stability of subgroups among adult dyslexic readers of a shallow orthography was explored by comparing three different cluster analyses based on previously suggested combinations of two variables. These were oral reading speed versus accuracy, word versus pseudoword reading speed, and phonological awareness versus rapid naming. The three analyses were conducted with the same group of dyslexic adults. Each analysis produced three subgroups, corresponding to ones previously suggested in the literature. However, the subgroups had only little overlap from one analysis to another. Each clustering produced somewhat different subgroup profiles in phonological processing, reading, intelligence, temporal acuity, and sensory short-term memory. However, the shared difficulties of the solutions in several language-related and sensory tasks suggest the conclusion that developmental dyslexia does not causally consist of subgroups, at least in shallow orthographies. Further, the shared sensory difficulties suggest that impaired temporal acuity and sensory short-term memory may reflect the severity of a primary disorder that dyslexic readers cannot compensate by strategies.