picture of overall student performance. When these assessments are used to make high-stakes decisions about individual students, the potential negative consequences are likely to fall most heavily on groups with special learning needs, such as students with disabilities.

More than 5 million students with disabilities participate in special education programs under the IDEA. They vary widely in the severity of disability, educational goals, and degree of involvement in the general education curriculum. Although federal legislation defines 13 categories of disability, 90 percent of all special education students have one of four disabilities: speech or language impairment, serious emotional disturbance, mental retardation and/or specific learning disabilities. This diversity has important implications for how students with disabilities participate in large-scale assessments: for example, some participate fully in ways that are indistinguishable from their general education peers, some require modifications or accommodations in the testing procedure, and others are exempted from participation entirely (National Research Council, 1997).

For a number of reasons, many students with disabilities have previously not been included in the large-scale assessment programs conducted by their states and districts (National Research Council, 1997). In order for some students with disabilities to participate, accommodations—such as braille versions, alternate settings, extended time, and calculators—will need to be provided during testing. The purpose of accommodations is to correct for the impact of a disability that is unrelated to the subject matter being tested; in essence, the disability interferes with the student's capacity to demonstrate what he or she truly knows about the subject (Willingham, 1988).

Validity will be improved when testing accommodations are designed to correct for distortions in scores caused by specific disabilities. However, accommodations should be independent of the construct being measured (Phillips, 1993, 1994; American Educational Research Association et al., 1985). Determining whether an accommodation is independent of the construct is difficult for some types of disability, especially cognitive disabilities. Moreover, there is little research on how to design accommodations, a problem that is exacerbated by the lack of a reliable taxonomy for describing disabilities. Some strategies, such as computer adaptive testing for students who need extra time, may accommodate a large share of students with special needs without threatening the validity of test results. However, more research and development—along with access to the technology—are needed to bring this and other strategies into widespread use.

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