Answer
Stratification divides a population into sub-populations (strata) with similar characteristics—commonly by value—so each is sampled separately. High-value items may be tested 100% while lower-value strata are sampled. This reduces variability within strata, improves efficiency, and focuses effort where misstatement would matter most.
Real-world example The auditor tests all balances over $100k individually and samples the remaining smaller balances separately.