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Some Measurement Concepts1
WHAT TYPES OF TEST SCORES ARE THERE?
Different types of scores provide different types of information and serve different purposes. You should understand the different types of scores before you can use them or select scores that are most appropriate for your needs.
In this section, we define these types of test scores:
• raw scores,
• total percentage correct scores,
• object mastery scores,
• percentile scores,
• stanine scores,
• grade equivalent scores,
• standard scores, and
• normal curve equivalent scores
and explain the advantages and disadvantages of each. In the next section, we discuss how to use them.
Remember that test scores reflect only what was measured on a particular test (its domain). For example, scores on the Iowa Tests of Basic Skills (ITBS) test of mathematics achievement reflect only the combination of skills tested by the ITBS. Scores on other mathematics tests are comparable to the extent that their domains are comparable.
Raw scores
Raw scores indicate the number of items a student answers correctly on a test. For students who take the same test, it makes sense to compare their raw scores. If one third grade student answers 12 of 25 items correctly and another answers 16 correctly, then we likely will conclude the second student knows the content of that test better than the first.
Because the number of items varies between tests and because tests vary in difficulty, raw scores have little value in making comparisons from one subject to another. Suppose a third grade student answers 12 out of 25 items correctly on a mathematics test and 16 out of 25 items on a reading test. Some people may assume that the student is better in reading than in mathematics. However, we really know nothing about relative performance in the two different areas because the mathematics test may be much harder than the reading test.
How Are Raw Scores Distributed?
As an example of how raw scores are usually distributed over the population, let's look at a national sample of 2,000 students.
1 WritteRnubdynLear,wLre.nacnedRWud.nSerchafer (2002) What Teachers Need to Know About Assessment. Washington, DC: National Education Association.
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