Has Rasch Scaling Made Better Tests?

"Has it helped us understand better what kids know?" I believe the answer to these questions is not "no" but a resounding "yes." Unfortunately, many test developers using IRT do not take advantage of the powerful interpretation features that become accessible when person abilities and item difficulties have been calibrated on a common Rasch scale. This is probably one reason that some observers have not appreciated some benefits of Rasch measurement.

Allow me to take a recent chapter I have written, and elaborate on its contents for those who may be interested:

Woodcock, R. W. (1999). What can Rasch-based scores convey about a person's test performance? In S. E. Embretson & S. L. Hershberger (Eds.), The new rules of measurement: What every psychologist and educator should know. Hillsdale, NJ: Lawrence Erlbaum Assoc.

The title of the chapter explains its purpose and why it is relevant to this topic. The chapter begins by describing four levels of information that are potentially available from any test and how the use of Rasch measurement helps exploit this available information. The chapter describes several procedures that facilitate interpretation of test performance including:

1) Rasch-scaled item maps.

2) The "Relative Proficiency Index" which states a prediction of the subject's level of success on tasks performed with 90% success by average age- or grade-mates. This index is similar to the Snellen Index used in vision examinations. An example is "67/90" which states that the subject is predicted to perform with 67% success those tasks that others perform with 90% success.

3) The "Developmental Zone" used in cognitive tests and the "Achievement Zone" used in achievement tests. These zones identify a range along an age or grade scale from the level at which the subject will find tasks "easy" (RPI = 96/90) to the level at which the subject will find tasks "difficult" (RPI = 75/90).

4) After some other examples, the chapter concludes with an example test report based on an assessment of Spanish and English language proficiency in a bilingual student. The scores and text highlighted by bold type in a figure in the chapter identify the information derived from Rasch-based scales that could not have been provided by traditional CTT procedures. Half of the information in the report, and probably the most important information for teachers and others concerned with the question of what this child knows and can do, appears in bold.

The Embretson & Hershberger book has a lot of good information and I recommend it to you.

Richard W. Woodcock

Has Rasch scaling made better tests? Woodcock R.W. … Rasch Measurement Transactions, 2000, 14:1 p.738


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Rasch Publications
Rasch Measurement Transactions (free, online) Rasch Measurement research papers (free, online) Probabilistic Models for Some Intelligence and Attainment Tests, Georg Rasch Applying the Rasch Model 3rd. Ed., Bond & Fox Best Test Design, Wright & Stone
Rating Scale Analysis, Wright & Masters Introduction to Rasch Measurement, E. Smith & R. Smith Introduction to Many-Facet Rasch Measurement, Thomas Eckes Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, George Engelhard, Jr. Statistical Analyses for Language Testers, Rita Green
Rasch Models: Foundations, Recent Developments, and Applications, Fischer & Molenaar Journal of Applied Measurement Rasch models for measurement, David Andrich Constructing Measures, Mark Wilson Rasch Analysis in the Human Sciences, Boone, Stave, Yale
in Spanish: Análisis de Rasch para todos, Agustín Tristán Mediciones, Posicionamientos y Diagnósticos Competitivos, Juan Ramón Oreja Rodríguez

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