[Rasch] Sample free
trevor.bond at jcu.edu.au
Wed Jul 23 15:58:32 EST 2008
What is meant by "Rasch is sample free"
B&F 2nd Chap 10
"Those enamored with traditional principles might be
more than a little skeptical of some claims made for the Rasch family of
models. Often, the most difficulty is expressed concerning the assertion that
the Rasch family of models produces what are commonly referred to as
estimates of item difficulties and “item-free” estimates of person abilities.
This is, of course, a direct consequence of the development of invariant,
fundamental measures: The use of the shorthand descriptors, “person-free”
and “item-free” estimates raises some eyebrows in social science circles.
Rasch measures can be seen as the natural consequence of two complementary
Rasch measurement principles. The first is the calculation of item difficulty
estimates that are independent of the distribution of abilities in the particular
group of persons for whom the items are appropriate. The complement is the
of person ability estimates that are independent of the distribution of difficulties
of the particular set of items used for the estimation. In that case, we
should prefer “person-distribution-free” over “person-free” and “item-
free” over “item-free” because some, rather tendentiously, seem to understand
“person-free” to mean that no empirical person responses are required.
B&F 2nd Chap 13
MEASUREMENT AND ITEM RESPONSE THEORY
Of the few textbooks that deal with the Rasch model, most tend to lump it
with two- and three-parameter models under the general heading, Item
Theories. In that context, the Rasch model is referred to as the one-parameter
response theory (IRT) model. But the 1-PL, 2-PL, 3-PL models are so called
the test items are characterized by one, two, or three parameters, and the
persons by a distribution. The persons are not individually parameterized as in
Rasch model. This has important implications for the concept of measurement
invariance, because those IRT models are not “person-distribution-free” (see
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