1. Introduction to the Technique
Stratified sampling is an internal audit technique where the population is divided into subgroups—or strata—based on characteristics such as value, risk, or type of transaction. Samples are then drawn from each subgroup, ensuring better representation and focus on higher-risk areas. This approach is more efficient than random sampling when some categories are riskier than others. At ACME Corp, Internal Auditor Auren uses stratified sampling to focus deeper audits on high-value procurement transactions while still reviewing a representative spread of all activity.

2. Practical Template for Stratified Sampling
A useful stratified sampling worksheet includes the following fields:

• Audit Objective (e.g., Assess purchasing compliance by value tier)

• Strata Definitions (e.g., Low: <$1,000, Medium: $1,001–$10,000, High: >$10,000)

• Population Size per Stratum

• Sample Size per Stratum

• Sampling Method (Random within stratum)

• Transaction ID and Description

• Test Result (Compliant/Non-compliant)

• Observations and Issues

Stratified sampling can be implemented using Excel filters and random number generation, or more advanced data analytics tools.

3. Practical Application at ACME Corp: Procurement Audit
CEO Liora tasks Auren with auditing the Procurement-to-Pay process, which handles thousands of vendor payments annually. Auren, in coordination with Project Manager Aven and Procurement Head Nina, decides to use stratified sampling based on transaction value—a risk-sensitive variable.

Auren defines three strata:

• Low-value (under $1,000): 3,800 transactions

• Mid-value ($1,001–$10,000): 1,200 transactions

• High-value (above $10,000): 220 transactions

Given the risk, he selects:

• 10 low-value transactions (0.26%)

• 20 mid-value transactions (1.67%)

• 25 high-value transactions (11.4%)

Upon testing:

• Low-value purchases mostly pass, with minor documentation gaps.

• Mid-value purchases show three instances of missing approvals.

• High-value transactions reveal two major control lapses, including a split purchase to avoid approval thresholds and one vendor added without background checks.

Auren concludes that risks and control issues increase with transaction size, validating the use of stratification. His report recommends revising approval workflows and strengthening vendor onboarding protocols for high-value procurements.

Stratified sampling enabled Auren to focus effort where it mattered most, ensuring a comprehensive yet efficient audit. For internal auditors, this technique strikes a balance between risk-based auditing and representation—turning volume into insight.