Surgical perspectives in gastrointestinal disease: A study of quality of life outcomes in esophageal, pancreatic, colon, and rectal cancers

Figure 1 To measure QOL, social, psychological and physical domains of illness are assessed using measurement scales (z). QOL (Q) is dependent on both subjective and objective items. For a questionnaire to be validated, Z must equal Q. (Reprinted with permission from Testa et al N Engl J Med 334: 835-840, 1996).