Can sample size in qualitative research be determined a priori?
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Date
2018-03-27Author
Sim, Julius
Saunders, Benjamin
Waterfield, Jackie
Kingstone, Tom
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Sim, J., Saunders, B., Waterfield, J. & Kingstone, T. (2018) Can sample size in qualitative research be determined a priori? International Journal of Social Research Methodology, 21(5), pp. 619-634.
Abstract
There has been considerable recent interest in methods of determining sample size
for qualitative research a priori, rather than through an adaptive approach such as saturation.
Extending previous literature in this area, we identify four distinct approaches to determining
sample size in this way: rules of thumb, conceptual models, numerical guidelines derived
from empirical studies, and statistical formulae. Through critical discussion of these
approaches, we argue that each embodies one or more questionable philosophical or
methodological assumptions, namely: a nave realist ontology; a focus on themes as
enumerable 'instances', rather than in more conceptual terms; an incompatibility with an
inductive approach to analysis; inappropriate statistical assumptions in the use of formulae;
and an unwarranted assumption of generality across qualitative methods. We conclude that,
whilst meeting certain practical demands, determining qualitative sample size a priori is an
inherently problematic approach, especially in more interpretive models of qualitative
research.