Key Elements of a Sampling Plan – Purposeful Sampling
Before beginning the examination phase, the audit’s scope, methodologies, and objectives are documented and reviewed with the audited organization. If purposeful sampling is to be used during the audit to gather evidence, then a sampling plan could be developed and given to the auditees before the examination phase. It is suggested that a good purposeful sampling plan should cover the following five key elements.
Key element 1: Population analysis
The plan could include the results of the population analysis conducted using the administrative data sets obtained from the auditees (including an assessment of the quality of the data sets). Whereas generalizable sampling sometimes requires a segmentation of the population in subgroups, this is not usually the case for purposeful sampling.
Key element 2: Objectives of sampling
The plan could indicate that a purposeful sampling approach will be used and explicitly state the sample’s specific objective. Auditors could specify the assumptions, assertions, or hypotheses that are being tested, and what type of information is required to either confirm or deny these.
Key element 3: Sampling method
Based on the type of information being sought and the assumptions being tested, auditors could select, and document in the plan, a specific sampling method or mix of methods that will allow for definitive conclusions. Determining the specific method or mix of methods of sampling should be a collective decision that is discussed at length within an audit team. This will ensure that a firm selection rationale is established and that the chosen approach will provide reliable interpretation of observations. More information on basic methods of sample selection for purposeful sampling is in Appendix 3.
Key element 4: Reportable findings
Before making any observations, auditors could state in their plan what conclusion and recommendations would be appropriate for various levels of error that might be observed. Any conclusion based on audit observations needs to clearly explain how it is supported by the sampling methodology.
Key element 5: Sample size
The plan could provide a rationale for the initial sample size and the criteria for ending the selection process or expanding the sample.
Sample size for purposeful sampling is not a straightforward calculation. Sample size depends on many factors, including the method of sampling, the importance of the subject matter, the redundancy of findings, and the clarity of results. Even initial findings can influence sample size. More information on determining sample size for purposeful sampling is in Appendix 4.
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