A procedure to assess the interpersonal competence of teachers was developed and validated. The procedure includes the use of the Questionnaire on Teacher Interaction (QTI), an observation instrument and a self-evaluation instrument. The concepts of construct-irrelevant variance and construct under-representation were used to explore the validity of the assessment procedure with regard to the noting of evidence and combining of evidence to attain an overall judgement by four separate assessors for the same beginning teacher. The validity of the assessment procedure was found to be satisfactory and the use of multiple assessment instruments to clearly have added value.