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The Graphical Display of Simulation Results With Applications to the Comparison of Robust IRT Estimators of Ability IRT

Thissen, David; Wainer, Howard
Publication Year:
Report Number:
RR-84-36, PSRTR-84-49
ETS Research Report
Document Type:
Page Count:
Subject/Key Words:
United States Air Force Human Resources Laboratory, Computer Software, Data Interpretation, Graphs, Item Response Theory (IRT), Simulation, XTREE (Computer Program)


Simulation studies of the performance of (potentially) robust statistical estimation produce large quantities of numbers in the form of performance indices of the various estimators under various conditions. This report presents a multivariate graphical display used to aid in the digestion of the plentiful results in a current study of Item Response Theory (IRT) ability estimators. Called XTREE, or the "Christmas Tree Glyph," the display has a vertical line or "trunk" with angled lines or "branches" for each estimator which display bias (left) and random variance (right). The sum of the two branch lengths represents the mean square error (MSE). Appendix A is a tabular presentation of the results to illustrate the data interpretation problem. Appendix B contains the XTREE computer programs (written in FORTRAN IV for a Honeywell DPS computer system and a Hewlett-Packard 7470A plotter). (AUTHOR/BS). (35pp.)

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