Sandelowski M. Rigor or rigor mortis: the problem of rigor in qualitative research revisited. Advances in Nursing Science. 1993;16(2):1-8.
Sandelowski M. "To be of use": enhancing the utility of qualitative research. Nursing Outlook. 1997;45(3):125-132.
Rolfe G. Validity, trustworthiness and rigour: Wuality and the idea of qualitative research. Journal of Advanced Nursing. 2006;53(3): 304-310.