Discrepancy Differences and Dyslexia

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This 2003 IDEA language amendment to the definition of learning disabilities departs significantly from the common practice in the 1980s and early 1990s to look at the difference between IQ (intelligence quotients) scores and academic achievement to determine the presence of a learning disability (LD). IQ testing was considered a routine component to the psychological portion of a comprehensive LD assessment. States used prescribed discrepancy formulas to establish LD norms following the testing. Additionally, Verbal IQ and Performance IQ differences were further reviewed as significant differences between the two (especially when performance IQ was high) were considered LD indicators. Profile patterns for various IQ subtests (e.g., on the WISC-IIIR) emerged with different population subgroups such as AVID (see Spafford, 1989) with female dyslexics (low arithmetic, coding, information, and digit span subtests).

There has been growing criticism in the new millennium of the use of discrepancy differences between IQ and achievement as critical indicators for learning disabilities. Rourke (1998) and other researchers have suggested in more recent times that the use of IQ profile patterns is also of little diagnostic utility for children with learning disabilities, attributing prior research findings to inconsistencies in the use of LD terminology and study designs.

Juan E. Jimenez, Linda S. Siegel, and Mercedes Rodrigo Lopez (2003) looked at the relationship between IQ and achievement in English-speaking Canadian and Spanish children and found differences in reading tasks as a function of Performance IQ in English but not in Spanish. These researchers point to the orthographies themselves as accounting for this finding. Spanish, with consistent orthographic features by nature, fosters the early use of phonological processing in early reading. English has a relatively inconsistent orthography where words are less reliably decoded with reference to phoneme-grapheme correspondences alone. Therefore, the lack of reading experience has greater impact on an opaque orthography such as English where IQ performance has been previously found to significantly correlate with reading achievement for LD children. Jimenez and his colleagues cite additional research that shows low IQ scores do not necessarily equate to lower reading achievement, raising the question, Should IQ scores be used as predictors of reading achievement?  Angiulli and Siegel (2003) recommend concentrating future research efforts on ascertaining patterns of achievement scores for LD children versus patterns of IQ scores. Future research will help to clarify this issue.