Teacher evaluation systems commonly rely on observation of teaching practice (OTP) by school principals. However, the value of OTP as evidence of teacher effectiveness depends on its psychometric quality. In this study, we address a key aspect of the psychometric quality of principals' OTP ratings. Specifically, we investigate the degree to which rating scale categories have a consistent interpretation across teaching episodes and practices. Results suggest that the 1,324 principals' use of the rating scale categories functioned as intended overall. However, we also found that the midpoint category is underutilized and that rating categories do not always reflect similar levels of teaching effectiveness across teaching episodes and practices. When such discrepancies occur, we cannot assume principals' ratings reflect a consistent level of teacher effectiveness within and across classrooms. This is a critical component of validity evidence that can inform the interpretation of OTP ratings and point to areas for improvement in both the rubrics and in principals' training for classroom observations.