1. Nunally, J., & Bernstein, I. (1994). Psychometric theory (3rd ed., p. 104). McGraw-Hill: New York.
2. Leidy, N., & Vernon, M. (2008). Perspectives on patient-reported outcomes. Content validity and qualitative research in a changing clinical trial environment. Pharmacoeconomics, 26(5), 363–370.
3. U.S. Department of Health and Human Services. (2008). Food and drug administration. Guidance for industry. Patient-reported outcome measures: Use in medical product development to support labeling claims. Rockville, MD. http://www.fda.gov/cder/guidance/index.htm .
4. Denzin, N., & Lincoln, Y. (Eds.). (2003). Collection and interpreting qualitative materials (2nd ed.). Thousand Oaks, CA: Sage.
5. Snape, D., & Spencer, L. (2004). The foundations of qualitative research. In J. Ritchie & J. Lewis (Eds.), Qualitative research practice: A guide for social science students and researchers (pp. 1–23). London: SAGE.