Multi-factor scale consolidation when theory is weak

As a practical matter, Spirituality and Quality of Life in the health sciences are usually measured separately. Theoretical foundations for this distinction, however, are not strong. In this research, an empirical investigation was conducted into their joint calibration with a Rasch model. Functional Assessment of Cancer Therapy-General (28 items), a cancer health-related quality of life measure (HRQOL), and Functional Assessment of Chronic Illness - Spiritual Well-Being (12 items), a measure of religious and existential well-being (Spirituality), were co-calibrated with a Rasch model implemented with WINSTEPS software for ratings from 545 breast cancer patients. The results show a hierarchical integration of QOL and Spirituality items on a common variable, and both patient separation (2.66) and reliability (.88) improve after co-calibration. Principal Component Analysis of co-calibrated item residuals did not show major threats to dimensionality, and joint calibration explains item variance comparable to separate calibrations (51.9%). Although patient measures (logits) based on separate and co-calibration are within two standard errors, ethnic and racial group values shift after co-calibration.