What is a dual-purpose test?
ISA 330.A23 recognises that it can be efficient to design a single procedure that addresses both a test of controls and a substantive test of details. Consider a purchase invoice: you can inspect whether the invoice was approved by an authorised person (test of controls for the approval control) and at the same time vouch the invoice amount, date, supplier, and delivery reference to the purchase order and goods received note (test of details for the occurrence and accuracy assertions). One sample, two conclusions.
But efficiency creates a design risk. The sample size for a test of controls is determined by the tolerable deviation rate and the expected deviation rate under ISA 530.A10. The sample size for a test of details is determined by the tolerable misstatement and the expected misstatement. These two calculations almost never produce the same number. ISA 330.A23 requires you to use the larger of the two sample sizes. In practice, the substantive sample is usually larger because it must provide sufficient evidence to detect misstatements at the assertion level.
The working paper must clearly separate the two evaluations. A dual-purpose test that only records "no exceptions found" without distinguishing whether the exception relates to the control element or the substantive element is undocumented under ISA 230.8. Each objective gets its own conclusion.
Key Points
- A dual-purpose test serves two objectives at once: control effectiveness and substantive assurance over the same transactions.
- ISA 330.A23 permits dual-purpose tests but requires the auditor to distinguish the two objectives clearly in the working papers.
- The sample size must satisfy both the test of controls requirements and the substantive testing requirements — use the larger number.
- If the control element fails, you lose reliance and must expand substantive testing for the entire population.
Why it matters in practice
Teams frequently use the test of controls sample size for the dual-purpose test, which is almost always smaller than the substantive requirement. ISA 330.A23 requires the sample to be sufficient for both purposes. If the substantive calculation demands 40 items and the controls calculation demands 25, the sample is 40.
Working papers frequently record a single conclusion for the dual-purpose test ("no exceptions noted") without separating the control evaluation from the substantive evaluation. When an exception relates to the control element but not the substantive element (or vice versa), the single-conclusion format obscures which objective is affected. ISA 230.8 requires documentation sufficient for an experienced auditor to understand the work performed, the results, and the conclusions.
Worked example: Galway Software Ltd
Client: Irish SaaS company, FY2024, revenue EUR 22M, IFRS reporter.
Galway licenses cloud-based project management software on annual subscription contracts. The engagement team designed a dual-purpose test over subscription revenue.
Identify the dual objectives: The control: Galway's revenue manager reviews and approves each new subscription contract before it enters the billing system, verifying the contract terms match the price list and the revenue recognition schedule. The substantive objective: test the occurrence and accuracy of recorded subscription revenue by vouching recorded revenue entries back to signed contracts.
Determine the sample size: The test of controls requires a sample of 25 items (based on tolerable deviation rate of 5%, no expected deviations, 90% confidence level). The substantive test requires 40 items (based on population of 1,840 subscription lines totalling EUR 22M, performance materiality of EUR 330K). The team selected the larger: 40 items.
Execute the test: For each of the 40 selected subscription lines, the team inspected: (a) the revenue manager's approval on the contract (test of controls element), and (b) the contract amount, billing period, start date, and revenue allocation schedule against the recorded revenue entry (substantive element).
Evaluate results separately: Control element: 39 of 40 contracts showed revenue manager approval. One contract (EUR 8,400 annual subscription, Client #IE-2219) showed approval by the sales director instead of the revenue manager. Deviation rate: 2.5% (1/40). Substantive element: all 40 revenue entries matched the underlying contract amounts, billing periods, and allocation schedules. Zero misstatements identified.
Conclusion: The dual-purpose test provided evidence on both objectives from a single sample. The control deviation (undocumented delegation) did not breach the tolerable deviation rate but generated a management letter point. The substantive results supported revenue occurrence and accuracy. Both conclusions are documented separately, which is defensible under ISA 330.A23 and ISA 230.8.
Key standard references
- ISA 330.A23: A test of a control may be designed to simultaneously serve as a test of details (dual-purpose test). The auditor must consider each objective separately.
- ISA 530.A10: Factors affecting sample size — the dual-purpose sample must satisfy both the controls and substantive calculations.
- ISA 230.8: Documentation must be sufficient for an experienced auditor to understand the nature, timing, extent, and results of procedures performed and the evidence obtained.
- ISA 330.7: The nature, timing, and extent of further audit procedures shall be responsive to the assessed risks of material misstatement.
Related terms
Related reading
Frequently asked questions
How is the sample size determined for a dual-purpose test?
The sample must satisfy both the test of controls and substantive requirements. ISA 330.A23 requires the larger of the two calculations.
Can a dual-purpose test have different conclusions for each element?
Yes. A control deviation and zero substantive misstatements is a valid outcome. The working paper must separate the two evaluations.