In our previous work, we established interference-based cut-points to differentiate moderate from mild, and severe from moderate levels of severity for 16 symptoms as reported by cancer patients undergoing chemotherapy. This work examines how consistent the differentiation is over time. If the levels of severity successfully differentiate interference scores over time, then anchor-based categories can be developed to evaluate patients' responses to the intervention. To test the differentiation of the interference scores by the three severity categories, data from two symptom management trials were used. Five hundred and ninety-nine patients were queried at each of the six contacts that occurred over eight weeks as to the severity of each of the 16 symptoms on the scale from 0 (not present) to 10 and the extent to which each symptom interfered with enjoyment of life, relationships with others, general daily activities and emotions. Longitudinal models that related interference scores to severity levels of symptoms were used. Differences among adjusted mean interference scores for mild, moderate, and severe levels were tested at each contact. Differences among interference-based severity categories were consistent over time and clinically important, and thus can be used to anchor changes in symptom severity.