[Rasch] Interpretation of Item Plot
Mike Linacre (RMT)
rmt at rasch.org
Sat Mar 24 19:43:27 EST 2007
Thank you for asking us about your plot. Its shape certainly prompts
further thought ....
In your five-category scale, categories 2 and 4 are a little below modal.
This means that the category intervals on the latent variable (considered
from a Rasch-Thurstone perspective) are slightly narrower than the .6
logits (or so) necessary for ordered Rasch-Andrich thresholds. But is the
rating scale defective? This depends on several considerations. Here are four:
1) Category meaning: Are the categories intended to be modal, or are
categories 2 and 4 intended to be transitional (lower probability) states?
2) Model fit: Do the observations in each category fit in those categories?
3) Category inference: Do the average measures (corresponding to the
observations in each category) advance with the categories?
4) Sampling variation: Is the non-modality of those two categories (this
time) merely an accident of the data, rarely to be repeated?
The plot does not tell us enough to know whether a 3-category rating scale
would function better (though it might), nor does it tell us which
collapsing of the categories would be optimal.
rmt at rasch.org
At 3/23/2007, Petroski, Greg wrote:
>The item is ordinal with responses 0 to 4 with larger values indicating
>greater ability. Note that two of the five catagories do not ever have
>the highest probability of being endorsed. Am I correct in interpreting
>this as suggesting a defective item -- one that might work as well with
>only three categories?
Editor, Rasch Measurement Transactions
rmt at rasch.org www.rasch.org/rmt/ Latest RMT: 20:4 Spring 2007
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Rasch