Mike Linacre (RMT)
rmt at rasch.org
Thu Jan 10 10:02:12 EST 2008
Thank you for your questions.
Hambleton is touching on these concepts: "the item difficulty parameter
estimate is statistically invariant (i.e., the same) irrespective of the
person sample distribution within a population", and similarly for the
person ability estimate and the distribution of items within a population
This is what Ben Wright meant by "sample-free" and "test-free" measurement.
It has been a feature of Rasch measurement from the beginning:
www.rasch.org/memo1.htm Ben Wright, 1967, "Sample-free Test Calibration and
It is the separability of parameters in Rasch models (first noticed by
Ragnar Frisch) that is the mathematical reason for the invariance of Rasch
estimates. Rasch, 1977, www.rasch.org/rmt/memo18.htm - "On Specific
Objectivity: An Attempt at Formalizing the Request for Generality and
Validity of Scientific Statements."
At 1/9/2008, you wrote:
>I read this in an IRT book (Hambelton, Swaminathan & Rogers, 1991).
>Is this what Rasch folks also believe in?
>Your silence gives me the impression that , this is not true.
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT: 21:2 Autumn 2007
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