Tuesday, November 2, 2010

Winsteps Person Most Unexpected Responses

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The Rasch model IRT test score analysis has become a “commonly used statistical procedure” wrapped in layers of mystery. By contrast, right mark (or count) scoring (RMS) analysis is traditionally evaluated by just looking directly at a table of marks bounded by student test scores and question difficulty values.

One way to audit the Rasch model is to compare IRT and RMS analysis printouts. Many show identical data. Other IRT printouts provide valuable insights not present in RMS analysis.

A test of 24 students by 24 questions  was scored with Ministep and with Power Up Plus (PUP). Passing was 75% on this nursing school test.

Winsteps prints out identical data in Table 17.1 Person Statistics. Student names  are even listed in the same order for students with the same score.

The data from four columns in Winsteps Table 17.1 are re-tabled into Winsteps Table 6.5 Most Unexpected Responses.

 These values are plotted in red on PUP Table 3.

Hall, with an estimated IRT ability measure of 3.44 is the highest scoring student to have missed a question, #21. Murta, with an estimated IRT ability measure of 0.58, is the next to the lowest scoring student, missing #11, and seven more. This ranking of unexpected responses is directly related to student test scores. This makes good sense.

Top students are not expected to make wrong marks. No student is expected to miss easy questions. These are basic expectations for IRT.

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