The best means for identifying a "specific learning disability" (SLD) has been an enduring problem for special education. The primary difficulty has been a lack of consensus about the proper way to operationalize the formal definition articulated in the Individuals with Disabilities Education Act (IDEA). To provide uniformity, the then U.S. Office of Education (1977) formalized discrepancy in rules and regulations delineating criteria for SLD identification. The discrepancy notion soon became the primary (and often sole) criterion for SLD identification (Mercer, Jordan, Alsopp, & Mercer, 1996).
The use of discrepancy as the primary criterion for SLD identification created a number of problems. A major difficulty involved the over-identification of SLD. The SLD population has increased about 150% to the point where it represents over 50% of the special education population and over 5% of all students in school. These increases are unparalleled and unwarranted, especially when viewed in relation to other high incidence, mild disabilities (i.e., mental retardation (MR) and emotional disturbance (ED)). For example, the MR numbers have declined significantly with MacMillan, Siperstein, and Gresham (1996) suggesting that the reason resides in the misclassification of students as LD who might previously have been classified as MR. Additionally, Wong (1996) suggested that teachers may have overgeneralized the SLD concept in an effort to provide special education services for a greater number of students experiencing school difficulties.
Besides over-identification, another problem is found in the very different numbers of students with LD identified across settings. The significant variability is seen, for example, across states where prevalence rates have been found to range from 2% to 7% (Coutinho, 1995). There is little rhyme or reason for these different rates, and it appears that they may primarily reflect a lack of consistency in identification procedures (Lester & Kelman, 1997). Forness (1985) demonstrated how policy changes in California led to a 156% gain in SLD with concomitant losses in the MR and ED populations. In contrast, far greater consistency in prevalence rates have been found for categories like hearing impairment and physical/multiple disability (Singer, Palfrey, Butter, & Walker, 1989).
The overgeneralization of the SLD concept and inconsistency in applying identification criteria has led to a confounding between SLD and low achievement (LA). Over time, a conventional wisdom emerged which suggested that there were no differences between students with SLD and students with low achievement (LA). The idea of limited SLD-LA differences was based primarily on the Minnesota Studies particularly that of Ysseldyke, Algozzine, Shinn, & McGue (1982) who found a large number of identical scores between SLD and LA subjects as well as a high percentage of overlap between scores. In a meta-analysis of the Minnesota data, however, Kavale, Fuchs, and Scruggs (1994) found that, "the lower achievement scores of the LD group are of a magnitude that distinguishes them from their LA counterparts" (pp. 74-75). Algozzine, Ysseldyke, and McGue (1995) disagreed and suggested that "because students with LD may be the lowest of a school's low achievers, they necessarily represent a group of people with qualitatively different needs . . ." (pp. 143-144). What Algozzine et al. failed to consider were findings showing minimal group differences in the cognitive domain which mean that, when compared, the SLD and LA groups "represent two distinct populations . . . defined by an ability-achievement distinction represented in a different achievement distribution but not in a different ability distribution" (Kavale, 1995, p. 146). Similarly, Fuchs, Fuchs, Mathes, Lipsey, and Roberts (2002) concluded that SLD-LA differences in the area of reading were substantive and reliable. Thus, although there were large numbers of students with SLD, questions about their identity surfaced and it became necessary to defend SLD as a distinct category of special education.
As a result of problems in SLD identification, the usefulness of discrepancy as the primary identification criterion for SLD has been called into question. To replace the discrepancy model, a response to intervention (RTI) model has been proposed (Vaughn & Fuchs, 2003). The RTI seeks to replace traditional psychometric methods of identification with a protocol that moves away from deficit models by emphasizing student outcomes. Gresham (2002), for example, argued that "children who fail to respond to empirically validated treatments implemented with integrity might be identified as LD" (p. 499). Thus, RTI is viewed as a viable alternative approach where identification and intervention are more closely linked, and the special education process can be conceptualized in terms of treatment validity (L. Fuchs & D. Fuchs, 1998).