The Presence and Impact of Local Item Dependence on Objective Structured Clinical Examinations Scores and the Potential Use of the Polytomous, Many-Facet Rasch Model

Abstract

Objective The purpose of this research project was to extend the research on the robustness of the dichotomous Rasch model to violations of the local independence assumption to the polytomous many-facet Rasch model (MFRM). Candidate scores from oral examinations and objective structured clinical examinations (OSCEs) have been shown to contain variance due to rater error/bias. If the MFRM is robust to local item dependence (LID), then the MFRM could theoretically be applied to medical OSCEs. Methods Five OSCEs were used in the study: 3 chiropractic licensure OSCEs and 2 nursing licensure OSCEs. Items were assigned to split-halves based on common stimulus. Split-half correlations were compared with Spearman-Brown estimates of reliability based on Cronbach α with all items contributing. Two- and 3-facet MFRM analyses were performed, first with individual items contributing and second with station totals contributing. Correlations were estimated between the 2 MFRM estimates. Results Cronbach α estimates with all items contributing were all very high (>.87). Spearman-Brown estimates were all considerably higher than split-half correlations. Correlations between MFRM by items and by stations were all very high (>.993). Conclusions The research project provided evidence that OSCEs violate the local item independence assumption. The project also showed that the MFRM is quite robust to such violations. The authors recommend that the MFRM be applied to OSCEs by station totals for estimates of candidate ability, and by items for item performance measures and quality control programs.