Ben Wright:
"If the difficulty of an item were not invariant over some
useful domain, then the term difficulty would have no
useful meaning. The inch on my wooden yardstick is invariant as
long as I don't use the yardstick inside the furnace and, of
course, am careful to (a) originate the zero end of the
yardstick at the place from which I want to measure a difference
and (b) align the yardstick parallel to the direction in which I
want to measure and (c) hold the yardstick still and (d) look
carefully, with my glasses on, to see as sharply as I can what inch
mark seems to be reached. And, when I am serious, I will replicate
this procedure several times first to exclude wild values and
finally to extract one value with an allowance for error which I
will then use as the measure."
M. Hubey:
"I think something will have to be done with this concept that
takes into account the fact that the brain/mind is not like
anything else. If the maximum weight I can lift with my single arm
is 70 lbs, it will also be the same the next day, and the day after
etc. It will take a long time before lifting that weight becomes
easier. But with problem-solving techniques it is instantaneous. If
I learn how to solve a particular kind of problem, no matter what
its degree of difficulty, it is a done-deal. Next time it is no
longer difficult."
Tom O'Neill:
"It seems that the person merely has more ability after learning
the new problem-solving technique. Having learned the technique
does not change the difficulty of that type of problem relative to
the other problems. If, however, there is no hierarchy of problems
and problem solving techniques are not acquired in any particular
order, then measurement is not possible. For example, if you can
perform long division, I assume that you can also do single digit
addition. This is because I envision a hierarchy of math
operations. When you violate this common understanding of math
ability, common because when calibrating various problems they
maintain the same relative difficulty, I must decide what to do
with your improbable response. Were you careless on the single
digit addition? Were you lucky on the long division? Does your math
ability conform to the common understanding of math ability?"
John Michael Linacre:
"In fact, the loss of invariance is major challenge to pre-post
test equating. If I can't read Chinese, at the pre-test, all
sentences in Chinese are equally difficult to read. When I can
read some Chinese, at the post-test, some sentences are easy and
some are hard. When I'm learning to drive a car, some things are
easy and some things are hard. When I've learned to drive, all
essential skills are equally easy. So where is the yardstick? We
have to decide which situation corresponds to our intention to
measure, and then use that situation to quantify the invariant
measures that we will apply everywhere. The metrologists follow
exactly this procedure when they construct physical measuring
instruments."
The Problem of Measure Invariance. Wright, B.D., Huber, M., O'Neill, T., Linacre, J.M. Rasch Measurement Transactions, 2000, 14:2 p.745
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