What is stratification in audit sampling?
ISA 530.A6 describes stratification as dividing a population into discrete sub-populations so that each stratum has similar characteristics. The practical effect is straightforward: a population with wide variability in item values requires a larger sample to achieve a given level of assurance. If you split that population into strata with narrower variability, each stratum requires a smaller sample, and the sum of those stratum-level samples is typically smaller than the single unstratified sample would have been.
The most common approach for tests of details is monetary stratification. Items above a certain threshold (often performance materiality or a percentage of it) go into a "test all" stratum. Items between that threshold and a lower cutoff form a middle stratum sampled at a moderate rate. Items below the lower cutoff form a residual stratum sampled lightly or excluded if their total value is immaterial.
ISA 530.A10(e) lists the stratification of the population as a factor the auditor must consider when determining sample size. This is not optional. If the population is highly variable and the auditor does not stratify, the resulting sample size must be large enough to compensate. Stratifying first is almost always more efficient.
Key Points
- Stratification reduces the variability within each sub-population, which allows a smaller total sample to achieve the same confidence level.
- The most common stratification criterion is monetary value, but risk characteristics and transaction type are also valid bases.
- ISA 530.A6 explicitly states that stratification can result in a lower sample size without a proportional increase in sampling risk.
- Poor stratification (or none at all) is one of the fastest ways to end up with an over-sized or under-targeted sample.
Why it matters in practice
Teams frequently stratify by value but fail to document why the stratum boundaries were set where they were. ISA 530.A6 requires the strata to have similar characteristics. A boundary at EUR 50K needs a rationale (e.g., "EUR 50K represents 2% of performance materiality" or "items above EUR 50K are individually significant to the assertion"). An arbitrary cutoff without documented reasoning is a review finding.
Some teams apply MUS to an unstratified population and assume the monetary weighting inherent in MUS is equivalent to stratification. It is not. MUS weights selection probability by value, but it does not ensure adequate coverage of the mid-range stratum. ISA 530.A6 treats stratification as a separate design step that precedes sample selection, not as a feature built into MUS.
Worked example: Transportes Iberica SL
Client: Spanish logistics company, FY2024, revenue EUR 96M, Spanish GAAP (PGC) reporter.
Population: 4,120 trade receivable balances totalling EUR 22.4M, tested for existence.
Step 1 — Analyse the population for variability: The team exports the receivables listing and sorts by value. 18 balances exceed EUR 200K each, totalling EUR 8.1M (36% of population value). The remaining 4,102 balances range from EUR 12 to EUR 195K, with a mean of EUR 3.5K and high variability.
Step 2 — Define strata: The team creates three strata:
- Stratum 1 (individually significant): all 18 items above EUR 200K. Tested 100%. No sampling.
- Stratum 2 (moderate value): 314 items between EUR 30K and EUR 200K, totalling EUR 9.8M. Sampled using MUS.
- Stratum 3 (low value): 3,788 items below EUR 30K, totalling EUR 4.5M. Sampled using MUS with a lower required assurance because Stratum 1 and 2 cover 80% of population value.
Step 3 — Calculate stratum-level sample sizes: Performance materiality is EUR 2.9M. Tolerable misstatement for the full test is EUR 2.9M, allocated across strata based on value:
- Stratum 1: 18 items tested in full (no sampling risk for these items).
- Stratum 2: TM allocation EUR 2.0M. At 95% confidence (R-factor 3.0), expected error EUR 50K: sample = (9,800,000 x 3.0) / 2,000,000 = 15 items.
- Stratum 3: TM allocation EUR 0.9M. At 90% confidence (R-factor 2.3), expected error EUR 20K: sample = (4,500,000 x 2.3) / 900,000 = 12 items.
Step 4 — Execute and evaluate per stratum: Each stratum is evaluated separately. One error of EUR 14K found in Stratum 2, projected within that stratum. No errors in Stratum 3. Stratum 1 items confirmed without exception. Total projected misstatement across all strata: EUR 73K, below PM of EUR 2.9M.
Conclusion: Stratification allowed the team to test 45 items while covering all high-value balances in full and sampling the mid-range and low-value strata separately. An unstratified MUS approach would have pulled predominantly high-value items, leaving the mid-range (where the actual error was found) under-represented.
Key standard references
- ISA 530.A6: Stratification guidance — dividing a population into sub-populations with similar characteristics to reduce variability and improve sample efficiency.
- ISA 530.A10(e): Stratification as a factor the auditor must consider when determining sample size for tests of details.
- ISA 530.A11: The auditor may use statistical or non-statistical methods for sample design, including stratified approaches.
Related terms
Related tools
Related reading
Frequently asked questions
Why does stratification reduce the required sample size?
By lowering variability within each stratum, the auditor can achieve the same confidence level with fewer items per stratum.
Is MUS the same as stratification?
No. MUS weights selection probability by value but does not ensure adequate coverage of mid-range items. Stratification is a separate design step that precedes sample selection.