Sampling in qualitative research. Purposeful and theoretical sampling; merging or clear boundaries?

Sampling is a very complex issue in qualitative research as there are many variations of qualitative sampling described in the literature and much confusion and overlapping of types of sampling, particularly in the case of purposeful and theoretical sampling, The terms purposeful and theoretical are viewed synonomously and used interchangeably in the literature. Many of the most frequent misinterpretations relate to the disparate meanings and usage of the terminology. It is important that the terminology is examined so that underlying assumptions be made more explicit. Lack of shared meanings and terminology in the nursing discourse creates confusion for the neophyte researcher and increases the production of studies with weak methodologies. This paper analyses critically purposeful and theoretical sampling and offers clarification on the use of theoretical sampling for nursing research. The aim is not to make prescriptive statements on sampling; rather, to enhance understanding of the differences between purposeful and theoretical sampling for nursing research.