Introduction of the Source of Systematic Variation

Introduction of the Source of Systematic Variation

When the characteristics or conditions of the assessment tasks are altered, as when test-takers are offered accommodations, another source of systematic variation is introduced. These alterations can yield results that are explained not by differences in what students know and can do, but by the accommodations themselves.

Moreover, when different groups of test-takers interact with different kinds of accommodations, the results may vary and thus these interactions can also constitute alternative explanations for performance. Therefore, in evaluating the effects of accommodations on the validity of inferences, three kinds of alternative explanations need to be considered: 1. Performance may be affected systematically by attributes of individuals associated with specific groups, such as students with disabilities and English language learners. 2. Performance may be affected systematically by accommodations, changes in the characteristics, or conditions of assessment tasks. 3. Performance may be affected systematically by the interactions between the attributes of groups of test-takers and characteristics of assessment tasks.

One crucial distinction that psychometricians make is between construct relevant and construct-irrelevant variance in test scores, that is, variance that is or is not related to the test-takers' status with respect to the construct being assessed (Messick, 1989). All three of these alternative explanations could be viewed as potential sources of construct-irrelevant variance in test performance. The case that an alternative explanation accounts for score variation would need to be based on rebuttal data that provides further details on the probable reasons for score variation. Seen in the context of a validation argument, the primary purpose of an assessment accommodation is to produce test results for which the possible alternative explanations, such as test-taker characteristics that are irrelevant to what is being measured, are weakened, while the intended inferences are not weakened. The fourth grade NAEP reading task discussed earlier can be used to demonstrate how one or more alternative explanations might work and the nature of the rebuttal data that might support them.

Suppose Tina is an English language learner whose family's cultural heritage is not North American and who is not familiar with some of the concepts presented in the passage, such as winter, fireplaces, and soapstones. In this case, it could be that Tina's lack of familiarity with some of the cultural elements of the passage, rather than inadequate reading proficiency, constitutes an alternative explanation for her performance; if this is true, it weakens the intended inference. It may well be that Tina's reading is better than the basic level described for fourth grade but that her lack of familiarity with important concepts prevented her from being able to answer the questions based on this passage correctly. The components of this argument are illustrated. For this part, learning a foreign language needs a leaning tools, many students choose Rosetta Stone Arabic and Rosetta Stone Chinese to learn Arabic and Chinese.

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