This article, but Louisa Moats and Carol Tolman, discussed the three main areas of reading disabilities: (1) phonological deficit, (2) processing speed/orthographic processing deficit and (3) comprehension deficit.
“For purposes of research, “reading impaired” children may be all those who score below the 30th percentile in basic reading skill. Among all of those poor readers, about 70-80 percent have trouble with accurate and fluent word recognition that originates with weaknesses in phonological processing, often in combination with fluency and comprehension problems. These students have obvious trouble learning sound-symbol correspondence, sounding out words, and spelling. The term dyslexic is most often applied to this group.”