The Rasch rating scale model was used to provide diagnostic information about a teacher effectiveness instrument, the Inventory of Myself as a Professional (IMP). Two forms of the instrument, a self-report form to be completed by the teacher and an observation form to be completed by a supervisor or administrator contained the same thirty-six items. The items were included on three subscales: (1) student to teacher relationships; (2) professional attitude; and (3) professional skills. Response patterns were scored on a scale from one to five. The effectiveness of 204 teachers as observed by their principals on the Observation Form of the IMP and the effectiveness of 211 teachers as reported on the Self-Report Form of the IMP were assessed. Data for both forms were collected for teachers from public elementary and secondary schools in Colorado. Both forms appeared to fit the Rasch model quite well. The teacher effectiveness variable was defined on both instruments, providing construct validity to the IMP. Item calibrations sufficiently spread the person measures to ensure an adequate fit of the model on both forms. Having person measures contributed to the diagnostic potential of the IMP. Results suggest that the Rasch model provides a major breakthrough in analyzing the variable of teacher effectiveness in the use of item calibrations on a hierarchical continuum. Instructions for both forms of the instrument are included. A table and six figures illustrate the study results. (SLD)