An internal auditor tested whether purchase orders were supported by appropriately approved purchase requisitions She...

Nonsampling risk is the risk that the auditor may reach incorrect conclusions for reasons not related to the sampling process, such as failure to recognize exceptions or misinterpretation of audit results. In this case, the internal auditor did not notice that some purchase requisitions were approved by unauthorized persons, which is an oversight unrelated to the sample size or selection process. This is distinct from sampling risk, which is the risk that the sample selected does not represent the population. References:

The IIA’s International Standards for the Professional Practice of Internal Auditing (Standards), Standard 2320 - Analysis and Evaluation.

The IIA’s Practice Guide on Audit Sampling.