Theoretical sampling refers to the process of choosing new research sites or cases to compare with ones that have already been studied. It is one of the tools of qualitative research. The term was coined by Barney Glaser and Anselm Strauss in 1967.
The goal of theoretical sampling is not the same as with the probabilistic sampling; the researcher's goal is not the representative capture of all possible variations, but to gain a deeper understanding of analysed cases and facilitate the development of analytic frame and concepts used in their research.
Theoretical sampling can be viewed as a technique of data triangulation: using independent pieces of information to get a better fix on something that is only partially known or understood.