Intra-individual differences models
A commonly proposed alternative to models based on aptitude-achievement discrepancies and/or low achievement involves an examination of individual differences on measures of cognitive function. Thus, for example, a recent consensus paper from 10 major advocacy groups organized by the National Center for Learning Disabilities (NCLD, 2002) stated that "while IQ tests do not measure or predict a student's response to instruction, measures of neuropsychological functioning and information processing could be included in evaluation protocols in ways that document the areas of strength and vulnerability needed to make informed decisions about eligibility for services, or more importantly, what services are needed. An essential characteristic of SLD is failure to achieve at a level of expected performance based upon the student's other abilities (p. 4)."
This statement proposes intra-individual differences as a marker for "unexpected underachievement." As opposed to a single marker like IQ-discrepancy or low achievement, unexpectedness is operationalized as unevenness in scores across multiple tests. The person identified as LD (by definition) has strengths in many areas of cognitive or neuropsychological function, but weaknesses in core attributes that lead to underachievement. LD is unexpected because the weaknesses lead to selected and narrow difficulties with achievement and adaptive functions. Proponents of this view believe that such approaches identify children as LD based on profiles across tests that differentiate types of LD, and also differentiate LD from other childhood disorders, such as mental retardation and behavioral disorders like attention-deficit/hyperactivity disorder (ADHD). This approach leads to definitions based on inclusionary criteria in which children are identified as LD based on characteristics that relate to intra-individual differences (Lyon et al., 2002).
A major assumption of this model is that identification based on performance patterns will lead to enhanced treatment of children with LD. The weakness of the model is the absence of evidence that strengths and weaknesses in processing skills are related to intervention outcomes. It is well-established that training in underlying processes does not usually generalize into the related academic area. For example, training on phonological awareness skills without a letter component produces gains in phonological awareness, but not in reading (National Reading Panel, 2000). There is also the issue of perpetuating identification models that have not resulted in better outcomes for children with LD, which could be termed "test and treat" models. The result could be even more testing of children for eligibility purposes, reinforcing the model currently in place. The scaling issues for this model are significant. If administration of IQ and achievement tests as part of a search for a two-test discrepancy is not well implemented in many schools (MacMillan & Siperstein, 2002), how well can a multi-test discrepancy model be implemented? It is difficult to fathom teaching this type of clinic model to school psychologists and educational diagnosticians across the country. Given the access to children in schools, why would a model often based on what is often a single exposure to a child in a lifetime be applied when the child could be seen many times within a school year as part of a longitudinal evaluation of progress? Finally, this approach to identification does not address the problem of children with relatively flat test profiles. Severity is correlated with unevenness due to the lack of independence of different tests that might be used to construct the profile (Morris et al., 1993). For example, children with either a severe reading or phonological awareness problem will show increasingly flat profiles in direct correspondence to severity. Thus, if the criterion is evidence of a discrepancy in neuropsychological or processing skills, such an approach may not exclude the most severely impaired children, irrespective of global measures like IQ, because more severely impaired children are less likely to show skill discrepancies due to the intercorrelation of the tests (Morris et al., 1993; 1998).
By definition, the intra-individual differences approach will produce unique subgroups of underachievers who vary in cognitive skills. Stanovich's (1988) phonological core- variable differences model, supported by the subtyping study of Morris et al. (1998), and Rourke's (1989) nonverbal learning disability model, are strong examples of the intra-individual differences model. One issue is independence on measures not used to form the classification. Keep in mind that Stanovich's model indicates that within the reading domain, language differences aren't expected. Rourke's model includes comparisons with children who have reading and math difficulties, showing characteristics of both NLD and verbal learning problems. The critical question is how such approaches lead to better outcomes for children with LD. The intra-individual differences model focuses on behaviors that are not directly related to intervention, such as processing skills (Torgesen, 2002). However, interactions beyond the interactions beyond the primary area of academic difficulty (word recognition, fluency, comprehension, and math) are hard to identify (Fletcher, Morris, & Lyon, 2003). Thus, the model has the most validity at the level of achievement markers, but simply collapses into a low achievement model in the absence of processing measures.
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