SLD and Discrepancy
According to Vaughn and Fuchs (2003), "At the heart of the controversy about [SLD] identification is the use of the IQ-achievement discrepancy" (p. 137). This is true only because RTI supporters appear to have exaggerated the deficiencies presumed to be associated with discrepancy. For example, one objection suggests that the degree of discrepancy demonstrated does not relate to severity level. This objection is rendered immaterial if discrepancy is properly viewed as a threshold concept documenting the presence or absence of underachievement, a necessary but not sufficient criterion for SLD identification. Discrepancy need not be related to severity to be useful in the identification process (Kavale, 1987).
Another objection suggests that the academic performance of students with a discrepancy does not differ from that of students without a discrepancy. This objection is based on the incorrect assumption that discrepancy necessarily has any bearing on academic performance. Students with and without a performance discrepancy may, in fact, possess the same level of low achievement and thus demonstrate similar academic performance. In a relative sense, both groups would appear to be "disabled" since both exhibit functional impairments in academic development. Keogh (1994) suggested that unexpected low achievement relative to ability is one of the basic elements defining SLD. The student demonstrating a discrepancy is different because that student may be properly termed as an underachiever and thus possess a primary feature of SLD.
If a student does not demonstrate significant underachievement, then the possibility exists that the student in question may fall into the category of "slow learner" (i.e., students with IQs from 70-85). About 14% of the school population falls in this IQ range and has been a long-standing problem. This segment of the school population has never been a special education category and probably never should be. A slow learner does not demonstrate unexpected low achievement but rather an achievement level consistent with IQ level (Gresham, MacMillan, & Bocian, 1996). Although such low achievement is problematic when there is a desire to leave no child behind, it nevertheless reflects a true state of affairs. What should not happen is a designation of SLD for a slow learner. The SLD concept should not be sacrificed to resolve a long-standing school problem. There may be solutions to the problems presented by the slow learner, but none should involve a loss of integrity for the SLD category.
The value of discrepancy lies in its ability to document the unexpected nature of the learning problem. Everything else being equal, there was little reason to believe that the particular student would experience learning difficulties. Since the underachievement dimension is integral to the SLD construct, it may represent a better "first gate to learning disabilities identification" (Speece, Case, & Malloy, 2003, p. 147) than the proposed RTI model. With a well below average achievement level, discrepancy indicates the presence of underachievement and a possible disability. Although the discrepancy concept is valid across IQ ranges, SLD should be associated only with significantly below average achievement levels. For example, Siegel (2003) lamented the fact that a student with an IQ of 130 and reading achievement score of 110 would be considered RD according to the discrepancy model. This is a specious argument since students are referred for evaluation only if they are exhibiting signs of academic difficulty. Gordon, Lewandowski, and Keiser (1999) warned against the use of the SLD label for "relatively well-functioning" students. There are few if any school districts that test all students and provide special services to any student with a significant IQ-achievement discrepancy. Special education should be provided when there is academic difficulty (i.e., below average achievement) and the criteria for special education classification are met. If the student described by Siegel was struggling educationally, then consideration might be given to other potential diagnoses such as Attention Deficit Disorder or Mood Disorder rather than blindly assuming a single diagnosis to explain complex behavior like poor academic performance. Conversely, it would be equally absurd to diagnose a child with an IQ of 50 and a reading score of 75 as RD and not MR.
Discrepancy models for SLD identification have also been criticized for presumably not yielding reliable information, but such an argument appears a bit contrived. The properties of different discrepancy models have been thoroughly evaluated, and a consensus emerged that standard score, regression methods were psychometrically defensible (Shepard, 1980; Wilson & Cone, 1984). Because students were often required to only meet the discrepancy criterion for SLD classification, any who did were, in fact, identified with a sound statistical procedure. Thus, discrepancy does provide reliable information (Reynolds, 1985). The real problem comes from large-scale studies showing that sometimes up to 50% of any SLD population does not meet the discrepancy criterion (e.g., Norman & Zigmond, 1980; Kavale & Reese, 1991; Shepard & Smith, 1983). This raises the question: Why were students who did not meet the discrepancy criterion identified as SLD? If a student does not meet a reliable criterion, then the resulting classification cannot be reliable. The problem has not been the reliability of the discrepancy criterion but rather the lack of rigor in its implementation: "public school practices for diagnosing children with LD bear little resemblance to what is prescribed in federal and state regulations (i.e., administrative definitions) defining LD" (MacMillan, Gresham, & Bocian, 1998, p. 323).
Arguably, the presence of measurement error in discrepancy model increases the risk of false negatives as well as producing some false positives. Measurement error only has an impact when there is rigid adherence to a single cut-point without further investigating the underlying processes and competing diagnostic hypotheses. For instance, if a discrepancy criterion of 15 points is used and a student has a 14 point discrepancy, that student may still be SLD just as a student with a 16 point discrepancy should not automatically become SLD. Clinical judgment would dictate that scores within a range of discrepancy, for example, between 10 and 20, be evaluated more thoroughly for other indicators of learning impairment such as family history, impaired phonemic awareness or phonological processing, slow or error-filled rapid automatic naming, poor vocabulary development, or limited working memory capacity. Diagnostic accuracy increases when multiple measures are used and provide a convergence of evidence for one diagnosis versus competing hypotheses.
Objection to discrepancy models for SLD identification have also included the criticism that they do not inform instruction, but this suggestion appears to miss the point that the real task is to first achieve reliable and valid classification. Discrepancy is best viewed as an identification criterion so there would be little reason to expect it to have any bearing on instructional decisions. Creating effective instruction can become the primary focus when identification procedures provide confidence that the student is "truly" SLD. It is unfortunate that special education has come to de-emphasize classification, thus creating a mindset where there is little concern about whether or not a student is "truly" SLD so long as effective instruction can be provided. Although effective instruction is the raison d'etre of special education, the system should insure that special education is provided only to those who require it. Unchecked advocacy will inexorably undermine the integrity of special education and the SLD category (Kavale & Forness, 1998).
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