[Rasch] Average Threshold Measure in PCM
Mike Linacre
rmt at rasch.org
Wed Mar 30 10:00:57 EST 2011
Matthew:
Communicating the functioning of a rating scale is challenging, especially
if your audience think of its categories as separate and equally-spaced
points on the latent variable.
If you want to communicate the categories as points, then the best points
for the intermediate categories are the locations on the latent variable at
which the probability of observing each category is the highest. These are
also the points where the expected score on the item is the category value.
In Winsteps these are the "AT CAT" measures in the ISFILE= output
file. These points are at infinity for the extreme categories, so Winsteps
reports the measures for expected scores of "lowest category + 0.25" ( =
CAT +0.25) and "highest category - 0.25" (= CAT - 0.25).
The Rasch item difficulty (in IFILE=) is the point on the latent variable
at which the highest and lowest categories are equally probable.
The Rasch-Thurstone thresholds (50%PRB in ISFILE=) dichotomize the rating
scale at each category boundary into 50% probability of being observed
below the category and 50% probability of being observed in or above the
category.
The points on the latent variable where the expected scores are 2.5, etc.,
are called the CAT-0.5 points in ISFILE=.
Does this help?
Mike L.
At 3/29/2011, you wrote:
>rasch'ers--
>interpretation of the threshold resulting from the partial credit model
>seems straightforward to me. however, while attempting to not overwhelm my
>audience i would like to present and discuss fewer parameters by using the
>average threshold ("average measure" in the winsteps IFILE). but how do i
>interpret this parameter? does it represent the point on the theta
>continue at which subjects have a 50% chance of being above/below the
>average score?
>for example, given an item scored 0-5 there are 5 thresholds (-2,-1,0,1,2)
>an average threshold = 0. does this mean that 0 is the theta value
>corresponding to an equal chance of scoring above or below 2.5 points on
>this item?
>
>thanks!
>
>--matthew
Mike Linacre
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT: 24:4 Spring 2011
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