Removing Rasch Misfit and Cleaning Windows

Question: I must remove nearly 50% of the items to meet the recommended fit criteria. What is wrong with my Rasch analysis?

Reply: Where does misfit end? Imagine you are cleaning a window.

First you see big dirty areas - so you remove the dirt.

Then you see smudges - so you wipe off the smudges.

Then you see scratches - so you polish out the scratches.

Then you see distortions - so you grind out the distortions.

Then you notice discolorations - so you .....

At each stage of a fit analysis, the data are changed so the definition of "good fit" changes. We stop when the measures are "good enough", in the same way as we stop when the window is "clean enough".

One way to verify that the measures are "good enough" is to output the person measures at each stage, and then to cross-plot the person measures for each stage against those of the previous stage. When the difference between the later stage and the earlier stage is too small to matter, or the person measures for a later stage are less meaningful than for an earlier stage (often because there are too few items remaining), then the measures for the earlier stage are "good enough".

Doing this "good enough" procedure, we may discover that the original misfit in the data is too small to have any meaningful impact on the measures. So that no editing of the data is needed.

Linacre J.M. (2010) Removing Rasch Misfit and Cleaning Windows. Rasch Measurement Transactions, 2010, 23:4, 1241

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