WASP (write a scientific paper): Sampling in qualitative research.

This paper outlines key concepts and considerations pertaining to sampling in qualitative research as part of a series of articles on qualitative research methods aimed at a medical and health sciences audience with a predominantly quantitative background. Planning of the sampling approach, including within-case sampling, multiple-case sampling, sample size considerations and qualitative sampling strategies are addressed. Concepts specific to the qualitative paradigm, including the iterative approach and saturation are discussed and a framework for evaluating the rigour of qualitative sampling strategies is presented.