Patients and Methods: A subjective significance questionnaire (SSQ), which asks patients about perceived changes in physical, emotional, and social functioning and in global quality of life (global QL) and the QLQ-C30 were completed by patients who received chemotherapy for either breast cancer or small-cell lung cancer (SCLC). In the SSQ, patients rated their perception of change since the last time they completed the QLQ-C30 using a 7-category scale that ranged from "much worse" through "no change" to "much better." For each category of change in the SSQ, the corresponding differences were calculated in QLQ-C30 mean scores and effect sizes were determined.

Results: For patients who indicated "no change" in the SSQ, the mean change in scores in the corresponding QLQ-C30 domains was not significantly different from 0. For patients who indicated "a little" change either for better or for worse, the mean change in scores was about 5 to 10; for "moderate" change, about 10 to 20; and for "very much" change, greater than 20. Effect sizes increased in concordance with increasing changes in SSQ ratings and QLQ-C30 scores.

Conclusion: The significance of changes in QLQ-C30 scores can be interpreted in terms of small, moderate, or large changes in quality of life as reported by patients in the SSQ. The magnitude of these changes also can be used to calculate the sample sizes required to detect a specified change in clinical trials. (C) 1998 by American Society of Clinical Oncology.