The objective of this methods project was to determine whether PFS is an outcome related to psychological well-being or QOL. It focused on the relationship between PFS, an outcome, and other outcomes of importance to patients. In contrast to a traditional comparison of interventions, the variable of interest, PFS, is somewhat problematic, because the presence or absence of progression can only be observed, precluding the ability to design a prospective randomized clinical trial. Defining the relationship between PFS and QOL or other patient-reported outcomes (PRO) involves considering data as if obtained from observational study. A model is provided, including potentially important causal and confounding relationships, for considering further study of the relation between PFS and QOL-related outcomes.