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Knowing What Students Know: The Science and Design of Eduacational Assessment
nents, that provide a variety of evidence to inform and support educational decision making.
PRECISION AND IMPRECISION IN ASSESSMENT
Assessments serve a vital role in providing information to help students, parents, teachers, administrators, and policy makers reach decisions. Sophisticated statistical methods have been developed to enhance the accuracy of assessments and describe precisely their margins of error. But the heightened, and possibly exaggerated, attention paid to standardized testing in the U.S. educational system can overshadow the essential point that even assessments meeting the highest technical requirements are still, by their nature, imprecise to some degree. As noted earlier, an assessment result is an estimate, based on samples of knowledge and performance from the much larger universe of everything a person knows and can do. Although assessment can provide valuable information about a student’s competence, scores may nevertheless vary for reasons unrelated to achievement, such as the specific content being assessed, the particular format of the assessment items, the timing and conditions for administering the assessment, or the health of the student on that particular day.
Educators assess students to learn about what they know and can do, but assessments do not offer a direct pipeline into a student’s mind. Assessing educational outcomes is not as straightforward as measuring height or weight; the attributes to be measured are mental representations and processes that are not outwardly visible. One must therefore draw inferences about what students know and can do on the basis of what one sees them say, do, or make in a handful of particular situations. What a student knows and what one observes a student doing are not the same thing. The two can be connected only through a chain of inference, which involves reasoning from what one knows and observes to form explanations, conclusions, or predictions, as discussed in the following section. Assessment users always reason in the presence of uncertainty; as a result, the information produced by an assessment is typically incomplete, inconclusive, and amenable to more than one explanation.
ASSESSMENT AS A PROCESS OF REASONING FROM EVIDENCE
An assessment is a tool designed to observe students’ behavior and produce data that can be used to draw reasonable inferences about what students know. In this report, the process of collecting evidence to support the types of inferences one wants to draw is referred to as reasoning from