<p>The measurement of interdisciplinarity has been approached through various methods, most notably by analyzing disciplinary diversity of references and co-authors. However, the disciplinary diversity based on co-authors has received little attention. By analyzing co-authors' publication histories, the study constructs three co-author-based interdisciplinary indicators that reflect disciplinary diversity of co-authors themselves and the differences in disciplinary backgrounds between them. A systematic comparison with traditional reference-based measures reveals that co-author-based and reference-based measures capture two related but fundamentally distinct dimensions of interdisciplinary research. Our findings reveal that co-author-based measures capture the collaborative diversity within research teams, while reference-based measures emphasize the scope of knowledge integration. This distinction highlights that a diverse team does not automatically produce work with broad knowledge integration, though it significantly increases the likelihood. The main analyses are conducted at a broad disciplinary level, and supplementary analyses at a finer level confirm that the observed patterns are consistent across different degrees of disciplinary granularity. The study argues that a comprehensive assessment of interdisciplinarity requires both co-author- and reference-based measures to capture the full spectrum of interdisciplinary research activities, from team formation to final output.</p>