Ontologies are integral components of many health information processing environments. Hence, quality assurance (QA) of the conceptual content of any ontology is essential. A methodology for facilitating the identification and correction of a kind of error called 'missing-role error' for ontology concepts is presented. Specifically, being a member of the 'null-role set' is used as an indicator that missing-role errors are likely. Our QA methodology was applied to the Biological Process hierarchy, one of the hierarchies of the National Cancer Institute thesaurus (NCIt), a prominent ontology in the biomedical domain. Two hypotheses pertaining to the effectiveness of our QA approach were investigated. Many missing-role errors were discovered and confirmed in our analysis. In comparison to a control sample, our QA approach yielded a statistically significant number of concepts with missing-role errors, confirming our two hypotheses. QA is a critical part of an ontology's life-cycle, and automated or semi-automated tools for supporting this process are invaluable. The presented approach is a useful addition to the arsenal of tools available to QA personnel.