Stratified sampling is a technique that involves dividing a population into subgroups (strata) that share similar characteristics and then taking a sample from each subgroup. In the context of testing purchase transactions by different procurement officers, the transactions can be stratified based on the officers, ensuring that the audit team selects a sample that represents each officer's transactions. This method helps in achieving a more accurate and reliable assessment of the procurement process across all officers.

The Institute of Internal Auditors (IIA), Practice Guide on Sampling

"Auditing and Assurance Services: An Integrated Approach" by Alvin A. Arens, Randal J. Elder, Mark S. Beasley