AU-C 520 requires auditors to perform analytical procedures at two mandatory points in every financial statement audit — during planning and near the end of the engagement — and sets specific conditions that must be met before analytical procedures can serve as substantive evidence for a financial statement assertion. Issued by the AICPA’s Auditing Standards Board, the standard governs how auditors build an expectation for a recorded amount, compare it to what the client actually reported, and investigate any significant unexplained difference.
What Counts as an Analytical Procedure
AU-C 520 defines analytical procedures as evaluations of financial information through the analysis of plausible relationships among both financial and nonfinancial data. The definition also covers the investigation of fluctuations or relationships that are inconsistent with other relevant information or that differ from expected values by a significant amount.1PCAOB. Comparison of Proposed AS 2305 With ISA 520 and AU-C Section 520 Investigation is not a separate step. It is part of what the standard means by an analytical procedure.
These procedures generally take one of four forms:
- Trend analysis, tracking an account balance across periods.
- Ratio analysis, examining relationships between accounts or between financial and nonfinancial data.
- Reasonableness testing, building a dollar expectation from underlying inputs such as headcount multiplied by average pay.
- Regression analysis, using statistical models with measurable precision and confidence levels.
The common thread is that the auditor never evaluates a number in isolation. Every procedure rests on a relationship, and the stronger and more predictable that relationship, the more useful the procedure.
When AU-C 520 Requires Analytical Procedures
The standard mandates analytical procedures at two points. Between them, the auditor may also elect to use analytical procedures as substantive tests, but that is a choice governed by additional requirements.
Planning-Stage Procedures
Analytical procedures during planning are not discretionary. Their purpose is to help the auditor understand the entity and identify areas where the risk of material misstatement is elevated, and they work alongside the risk assessment requirements of AU-C 315.
These procedures are typically broad. The auditor might compare the current trial balance to the prior year’s, scan key ratios against industry benchmarks, or review quarterly revenue for unusual spikes. The point is pattern recognition, not precision. Inventory growing 40% while sales drop 10% is worth investigating even before the exact figures are pinned down.
The results feed the audit plan. Deterioration in receivable turnover typically prompts expanded testing of the allowance for doubtful accounts. Revenue trends consistent with prior years and industry norms may justify a lighter substantive approach. The precision threshold at planning is deliberately lower than what substantive testing requires, because the auditor is mapping risk rather than proving balances.
Final Review Procedures
AU-C 520 also requires the auditor to perform analytical procedures near the end of the audit to help form an overall conclusion about whether the financial statements are consistent with the auditor’s understanding of the entity.1PCAOB. Comparison of Proposed AS 2305 With ISA 520 and AU-C Section 520
By this stage, the auditor has completed most fieldwork and built a detailed understanding of the client’s operations, risks, and accounting. The final review is a sanity check against that understanding. The auditor looks at consolidated balances, key ratios, and trends for anything that seems inconsistent with what fieldwork revealed.
If something unexpected surfaces this late, the auditor cannot note it and move on. The standard requires investigation through inquiry of management and, when necessary, additional audit procedures. An unexplained jump in deferred revenue caught during the final review may mean re-opening substantive testing on that balance even with the report deadline approaching. The timing is inconvenient by design; the final review exists to catch what earlier work missed.
Using Analytical Procedures as Substantive Evidence
When an auditor relies on an analytical procedure as substantive evidence about an assertion, four conditions have to be met. The bar is considerably higher than at planning. One limitation to note up front: for assertions with significant risks of material misstatement, substantive analytical procedures alone are unlikely to provide sufficient evidence, and analytical procedures are generally not well-suited to detecting fraud, because management override can produce artificial relationships that look reasonable.2Public Company Accounting Oversight Board. AS 2305 Substantive Analytical Procedures
Suitability for the Assertion
Not every account lends itself to analytical testing. The relationship being analyzed has to be stable and predictable enough to produce meaningful results. Payroll expense for a company with steady headcount and known rates is a strong candidate. A complex litigation accrual, driven by judgment, is not. Analytical procedures work best where a misstatement would not be visible from any individual transaction but would show up as an anomaly at the aggregate level.2Public Company Accounting Oversight Board. AS 2305 Substantive Analytical Procedures
Reliability of the Data
The expectation is only as good as the data behind it. AU-C 520 requires the auditor to assess data reliability based on its source, the controls over its preparation, and whether those controls have been tested. Data from an independent external source generally carries more weight than internally generated data. Internal data produced by systems with strong, tested controls is more reliable than data from systems the auditor has not evaluated.
Before relying on data for a substantive analytical procedure, the auditor must either test the controls over that data or perform alternative procedures to confirm the data is complete and accurate.2Public Company Accounting Oversight Board. AS 2305 Substantive Analytical Procedures Skipping this step is a common weakness: if the revenue expectation is built on unverified internal production data, the procedure does not carry much weight. When the client outsources significant processes, the auditor may need to review a SOC 1 report covering the service organization’s controls before relying on data that flows from that organization.
Precision of the Expectation
Precision refers to how closely the expectation approximates the true amount assuming no misstatement exists. A tighter expectation narrows the range of acceptable outcomes, making a material misstatement more likely to produce a noticeable difference. Saying revenue should be “somewhere around last year’s level” catches almost nothing.
Disaggregation improves precision. Testing revenue by product line and month yields a tighter expectation than testing total annual revenue. The predictability of the relationship matters too. Income statement accounts tied to operational drivers tend to be more predictable than balance sheet accounts, which is why analytical procedures see more use on the income statement. Regression models with quantifiable confidence intervals sit at the high end of the precision spectrum; simple trend comparisons sit at the low end.
Threshold for Acceptable Difference
Before running the comparison, the auditor must define a threshold: the maximum difference between the expected and recorded amounts that can exist without triggering further investigation. This threshold must be set at or below the tolerable misstatement for the account being tested.1PCAOB. Comparison of Proposed AS 2305 With ISA 520 and AU-C Section 520
Set the threshold too wide and misstatements slip through. Set it too narrow and every account flags a difference, generating follow-up work that adds cost without improving quality. The threshold should reflect the risk that a difference represents a misstatement that could be material on its own or when combined with others in the same account. If the recorded amount falls within the threshold, the procedure supports the conclusion that the assertion is fairly stated. If it falls outside, the auditor has a significant difference that requires investigation.
Investigating Significant Differences
AU-C 520, paragraph .07, lays out a two-step process when any analytical procedure identifies a fluctuation or relationship that differs from the expected value by a significant amount. The auditor must first inquire of management and obtain audit evidence relevant to their responses, and then perform other audit procedures as necessary.1PCAOB. Comparison of Proposed AS 2305 With ISA 520 and AU-C Section 520
The word to focus on is evidence. Accepting management’s explanation at face value does not satisfy the standard. If management attributes a 35% jump in receivables to a strong fourth quarter, the auditor needs sales reports, customer contracts, and shipping records that back that up. When the corroborating evidence confirms the explanation, the auditor can conclude the difference is not a misstatement. When it does not, or when management cannot explain the difference coherently, the auditor has to pivot to tests of details: individual invoices, recalculated accruals, third-party confirmations. The analytical procedure has failed to deliver the needed assurance, and the audit program changes accordingly.
Documentation the Standard Requires
AU-C 520 requires specific documentation when substantive analytical procedures are performed. The auditor must document the expectation developed, the factors considered in building it, and the results of comparing the expectation to the recorded amount.2Public Company Accounting Oversight Board. AS 2305 Substantive Analytical Procedures
Weak documentation is one of the most common deficiencies in this area. A workpaper that reads “expectation: consistent with prior year” has not documented an expectation. The file should show the data used, its source, why it is reliable, the mathematical or logical basis for the expected amount, the threshold that was set, and the conclusion reached. When a significant difference triggered investigation, the file needs the specific inquiries made, the corroborating evidence obtained, and the auditor’s evaluation of whether a misstatement exists.
Planning-stage and final-review procedures also require documentation, though the standard allows more flexibility. At minimum, the file should reflect the procedures performed, the results, and, for the final review, how those results affected the overall conclusion about the financial statements.
How AU-C 520 Differs From PCAOB AS 2305
AU-C 520 applies to audits of non-public entities under AICPA standards. Auditors of SEC-reporting public companies follow the PCAOB’s standards, including AS 2305, which covers the same ground with some notable differences. The distinction matters for anyone whose practice spans both.
The core mechanics align: develop an expectation, set a threshold, investigate differences that exceed it. AS 2305 is more prescriptive in two respects. It explicitly prohibits developing the expectation using the company’s own recorded amount or information derived from it. And it specifies that when investigating an exceeded threshold, the auditor’s procedures should extend beyond inquiry, making the corroboration requirement unmistakable.1PCAOB. Comparison of Proposed AS 2305 With ISA 520 and AU-C Section 520
AS 2305 also adds a step that AU-C 520 does not spell out as clearly: if the auditor investigates a difference and still cannot determine whether a misstatement exists, the auditor must evaluate the effect on the audit, including whether to revise the risk assessment and what additional procedures are needed.1PCAOB. Comparison of Proposed AS 2305 With ISA 520 and AU-C Section 520 Applying the AU-C 520 approach to a public company audit will not satisfy the PCAOB.