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What is the limitation of normal curve equivalent scores?
Normal curve equivalent scores do not give you easily understood information about an individual student's achievement level, unless they are compared to another value or are converted to a percentile score.
HOW SHOULD YOU USE TEST SCORES? Interpreting norm-referenced test scores
Normative test scores -- stanines, percentile scores, scaled scores, and grade equivalent scores -- measure an individual student's achievement in relation to the achievement of one or more large groups of students who took the same test. The comparison group may be composed of other students in your district or of students from a nationally representative sample. Thus, scores on norm-referenced tests are meaningful only in relationship to a comparison group.
Your school or district is not completely like the normative group. No district is. In many cases, the differences are minor and inconsequential. However, in other cases, schools can be so different that the national norms provided by the publisher do not accurately reflect school performance. Norms become less meaningful as your students and your testing program become more unlike the standardization sample.
If your students are tested at a different time of the year than the norm group was tested, the interpretation of the percentile score is unclear. For example, the CAT is normed in October. That means that you must give it in October to make your students' scores most meaningful. If you give the CAT in January, you cannot know if a student who scores in the 55th percentile is above or below average when compared to grade-level peers. (See the Appendix called Communicating a complete report card for your school for a list of the many ways in which your students, schools, and district may be different from the normative sample.)
Many of these differences can seriously affect your scores. This does not mean the national norms are useless; it means that you must evaluate the norms in perspective. Some publishers extrapolate norms so they are based on the week the test was given, for example. Norms give you an index of how well students perform on certain tasks -- tasks the test publishers have identified as representing the skills taught to the comparison group at the time the test was developed.
Norm groups are used at varying points in time but their data are actually historical. Scores that are above average, for example, may be only above the average of students in the norm group who were tested four years ago. They may not be above today's average for a similarly defined group..
The comparative baseline of norm-referenced tests is a powerful tool. In addition to worrying whether your Chapter 1 students are learning basic skills, for example, you probably are also interested in how well they are doing in relation to the nation. Although
Rudner, L. and W. Schafer (2002) What Teachers Need to Know About Assessment. Washington, DC: National Education Association.
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