How Many Interviews Are Enough?

TLDR

Guidelines for nonprobabilistic sample sizes are virtually nonexistent, and while purposive sampling relies on the concept of saturation, this idea offers little practical guidance for estimating required interview numbers before data collection. The authors used data from sixty in-depth interviews with women in two West African countries to systematically document how saturation and variability evolve during thematic analysis. They operationalized saturation and, based on the data, recommended that nonprobabilistic interview samples reach saturation within the first twelve interviews, with core themes emerging as early as six, and observed similar patterns of variability.

Abstract

Guidelines for determining nonprobabilistic sample sizes are virtually nonexistent. Purposive samples are the most commonly used form of nonprobabilistic sampling, and their size typically relies on the concept of “saturation,” or the point at which no new information or themes are observed in the data. Although the idea of saturation is helpful at the conceptual level, it provides little practical guidance for estimating sample sizes, prior to data collection, necessary for conducting quality research. Using data from a study involving sixty in-depth interviews with women in two West African countries, the authors systematically document the degree of data saturation and variability over the course of thematic analysis. They operationalize saturation and make evidence-based recommendations regarding nonprobabilistic sample sizes for interviews. Based on the data set, they found that saturation occurred within the first twelve interviews, although basic elements for metathemes were present as early as six interviews. Variability within the data followed similar patterns.