Low achievement models

Models based on the use of achievement markers can be shown to have a great deal of validity (see Fletcher et al., 2002; 2003; Siegel, 1992). In this respect, if groups are formed such that it is ensured that the participants do not meet criteria for mental retardation and have achievement scores that are below the 25th percentile, a variety of comparisons show that subgroups of underachievers emerge that can be validly differentiated on external variables and help demonstrate the viability of the construct of LD. For example, if children with reading and math disabilities are compared to typical achievers, it is possible to show that these three groups display different cognitive correlates. In addition, neurobiological studies show that these groups differ both in the neural correlates on reading and math tasks as well as the heritability of reading and math disorders (Lyon et al., 2002). These achievement subgroups, which by definition include children who meet either low achievement or IQ-discrepancy criteria, even differ in response to intervention, providing strong evidence for "aptitude by treatment" interactions: obviously we don't provide math interventions for children with only reading problems, and vice versa.

Despite this evidence for validity, many squirm when definitions based solely on achievement cutpoints are proposed, and for good reason. Simply utilizing a low achievement definition, even when different exclusionary criteria are applied, doesn't really operationalize the true meaning of unexpected underachievement. Although such an approach to identification is deceptively simple, it is arguable whether the subgroups that remain represent a unique group of underachievers. For example, how well are underachievers whose underachievement is attributed to LD differentiated from underachievers whose underachievement is attributed to emotional disturbance, economic disadvantage, or inadequate instruction (Lyon et al., 2001)? To use the example of word recognition, there is little evidence that these subgroups vary in terms of phonological awareness or other language tasks, response to intervention, or even neuroimaging correlates. In this respect, the validity is weak. Finally, as Francis et al. (in press) demonstrated, there are still major issues with the reliability of low achievement identification models, particularly when they are based on a single assessment.

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