You set overall materiality at $500,000 and design every test to catch a single error of that size. Three independent misstatements of $180,000 slip through, none triggering investigation. Together they total $540,000 and your opinion is wrong. Performance materiality is the buffer that prevents this exact failure.
HIGH-FREQUENCY: Aggregation risk is the chance that individually immaterial misstatements combine to exceed overall materiality. Sampling cannot eliminate it: every sample is an estimate, and undetected misstatements live in the unsampled population. If you tested at overall materiality, a misstatement just under that line in five accounts would clear every test and still bust the audit. Performance materiality tightens the per-test threshold so the cumulative ceiling stays intact, and it drives the nature, timing, and extent of further audit procedures.
KEY: Overall materiality is the user's threshold. Performance materiality is the auditor's working threshold. The gap is your cushion.
AU-C 320 does not prescribe a fixed number. Push toward 50% when controls are weak, the entity is new to you, prior-year misstatements were material, or the population is heterogeneous.
Common mistakes
- Setting performance materiality at 100% of overall. With both at $400,000, three misstatements of $140,000 each total $420,000, none individually flagged. Trap: choice that says "set performance materiality at the same level as overall."
- Confusing clearly trivial with immaterial. A $25,000 error on $500,000 overall materiality is immaterial individually but must be accumulated if it exceeds the clearly trivial threshold of, say, $15,000. Trap: "no further action required because well below overall."
- Forgetting projected misstatement. A $8,000 error in a $3M sample from $30M projects to $80,000. SUD entry is $80,000, not $8,000.
Bottom line
- Performance materiality equals 50% to 75% of overall (financial statement) materiality, set to absorb aggregation risk
- Tolerable misstatement is performance materiality at the account level; the sum across accounts can exceed overall materiality
- Clearly trivial threshold (typically 5% of performance materiality) is the de minimis floor; below it, do not accumulate
- Accumulate every non-trivial misstatement (factual, judgmental, and projected) and compare the total to overall materiality before issuing the opinion
Exam shortcut
When overall materiality is given without risk context, default to 60, 65%. Strong controls and clean history → push to 75%; weak controls, prior misstatements, or a new client → push to 50%. Trap: any choice that sets performance materiality at or above overall. Eliminate it instantly. For accumulation questions, projected misstatement from sampling (not the raw sample error) lands on the SUD.
The full lesson (about 1,644 words, 11 min read) adds 2 worked examples, all 6 common mistakes, a self-check, free in the app.
Learning objectives
- II.D2
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