The Effects of Person-Item Mismatches on the Integrity of the Item Characteristic Curve.

In 1990 a routine study confirming the accuracy of the Rasch model in predicting item difficulties led to an in-depth analysis of the impact of guessing on the Rasch model when used with multiple-choice items. This paper reviews the highlights of that research. Seventeen linked vocabulary tests of 110 items were each administered to groups of 400 to 600 elementary school students. The obtained data were then analyzed using the BIGSTEPS computer program (B. Wright and M. Linacre, 1992) in order to obtain item difficulties and person ability estimates for the 1,304 items and 7,711 students. The actual performance of each person-item interaction was compared to predicted performance at each interaction point in order to construct an empirical item characteristic curve. The empirical curve was compared to the theoretical Rasch based curve and the discrepancies noted. When restrictions were placed on the analysis, the differences between the empirically derived curve and the theoretical curve were decreased. (Contains two figures, two tables, and five references.) (Author/SLD)