Rejecting Best Items?

"I would suggest rethinking your reliance on Rasch fit statistics as a criterion for item rejection .... In many case, the best (most highly discriminating) items would be rejected if one relied on Infit and Outfit statistics."
NCME reviewer, as reported by Ryan Bowles.

Conventional wisdom says "When items correlate highly with one another, those with the highest average correlations are the best items" (Jm Nunnally, Psychometric Theory, 1967, p. 261). But it is well-established that there can be too much of a good thing ... inter-item correlations can become too high:

"Other things being equal, interdependent items tend to decrease the reliability of a test. ... For the tendency becomes to answer neither item or both items and thereby produces an effect equivalent to reducing the number of items in a test." (Percival M. Symonds, Factors influencing test reliability, Journal of Educational Psychology, 1928, 19, 73-87. Italics his.)

Rasch Infit and Outfit statistics flag items to which responses are overly predictable, an indication that, in some way, they are interdependent with other items.

Rejecting Best Items? R. Bowles … Rasch Measurement Transactions, 2003, 17:1, 917.

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Rating Scale Analysis, Wright & Masters	Introduction to Rasch Measurement, E. Smith & R. Smith	Introduction to Many-Facet Rasch Measurement, Thomas Eckes	Invariant Measurement: Using Rasch Models in the Social, Behavioral, and Health Sciences, George Engelhard, Jr.	Statistical Analyses for Language Testers, Rita Green
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