Appendix 4 – Sample Size Determination for Purposeful Sampling
Unlike in generalizable sampling, purposeful sampling is not supported by recognized mathematical criteria for predicting the minimum sample size that is necessary for reliable results. Obviously, for single-case sampling, sample size is not an issue. However, for other purposeful sampling methods, selecting criteria for how many cases need to be examined is important.
The most often cited criteria for sample size in qualitative research are those of saturation and redundancy. The idea is that the sample size should not be predetermined but should be allowed to be flexible and emergent. As more and more cases are examined, the level of insight and knowledge gained reaches a point of saturation and any further examination becomes redundant. However, this iterative strategy can create some challenges with respect to the budgeting and planning of audits.
Regardless of any decisions made regarding sample size, it is important to avoid judging the credibility or importance of findings based on the number of cases examined as is done with generalizable samples. Purposeful sampling is a trade-off of wider breadth for greater depth of understanding. As Patton (2015) states: “The validity, meaningfulness, and insights generated from qualitative inquiry have more to do with the information richness of the cases selected and the observational/analytical capacity of the researcher than with sample size.”
As a rule of thumb, this Practice Guide suggests starting with a small sample and then increasing the sample if no uniformity of information is achieved after the initial sample is reviewed. There is no correct or universally recognized method for calculating a sample size for purposeful sampling. Some offices or firms may come up with an arbitrary number, but it is only that—arbitrary. There is no mathematical formula to justify it. What matters for auditors is to have a sample that could be defended by demonstrating that it is reasonable and sensible. They could use the rationale for selecting the various types of purposeful sampling approaches suggested in Appendix 3 as the basis to explain how the sample was selected.
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