Learn how IDOR sampling works, why projections can increase assessments, and what you can do to evaluate a sample plan.
If you are undergoing an Illinois sales tax audit, one of the most important questions is not simply “What did we do wrong?” but whether the Illinois Department of Revenue (IDOR) will apply sampling and extrapolation methods.
Sampling is often the point at which an audit shifts from manageable to significantly more costly. A limited number of unsupported transactions identified within a short test period may be converted into an error rate and then projected across multiple months or years. These projected results frequently form the basis of the final assessment.
IDOR guidance indicates that taxpayers are required to maintain records supporting line-by-line items reported on returns, and that the Department may request records either in full detail or through a sampling approach. IDOR audit procedures also reference specific notices issued when a sampling method is used, including communications that outline the sampling methodology.
This guide explains the sampling process in clear terms, highlights areas to monitor during an audit, and outlines practical steps businesses can take to evaluate and respond to a proposed sample plan.
What sampling means in an Illinois sales tax audit
In an Illinois sales tax audit, IDOR may review individual transactions to verify compliance. But for businesses with high transaction volumes, reviewing every record is often impractical. In these cases, IDOR may apply a sampling method by selecting a representative portion of transactions or audit periods, identifying any errors within that sample, and projecting the results across the full audit population.
IDOR’s general audit information letter tells taxpayers that it may require records in samples or in detail, and that audit methods include testing and detailed reviews of source documents and general ledger accounts. That language matters because it signals that sampling is a standard part of audit procedures, especially where volume is high.
While sampling is not automatically unfair, it becomes problematic when the sample is not representative, when documentation is incomplete during the sample period, or when the projection method assumes every future month looks like the sample month.
The sample population and why it controls everything
Before you can determine whether a sample plan is fair, you need to understand what IDOR considers the population. The population is the set of transactions the auditor identifies as representing the audited activity.
Examples of populations IDOR might sample:
- All taxable sales for the audit period
- All non-taxed or “exempt” sales for the audit period
- A subset like resale transactions, out-of-state sales, or specific product categories
- Purchases that may create use tax exposure (common in Illinois sales tax audit reviews)
A key risk to watch: If the population is defined too broadly, your sample may overstate exposure. If it is defined too narrowly, the auditor may miss context that supports your position.
IDOR’s audit manual materials indicate that sampling plans can be generated through a Random Sample Generator case and that the Department issues a specific notice to inform taxpayers of its intent to use a sample and provide information about the sampling methodology.
If you have received a sample plan letter or your auditor mentioned sampling, you can create a free account with Sales Tax Helper to review how the audit population was defined and identify where the plan may be overstating the population.
IDOR sampling plan letters you may see
IDOR’s Sales Tax Audit Manual describes several audit letters connected to sampling and computer-assisted auditing.
Notice of Audit Population Sample Plan
IDOR’s manual describes the EDA-RR-83C, Notice of Audit Population Sample Plan, as a letter given to taxpayers informing them of the intent to use a sample and providing information about the sampling methodology that will be used.
Proposed sample plan agreement request
The manual also describes IDOR-8-CAA2, Audit Proposed Sample Report, as a letter that notifies the taxpayer of a proposed sample plan and requests the taxpayer to indicate agreement. The manual notes the taxpayer may accept or decline the proposed plan by email or MyTax.
This is an important, practical moment. It signals that the Department expects a response and recognizes that agreement is not automatic.
Computer-assisted audit data request
The manual also references a letter used to request data when the Computer Assisted Auditing Group is involved (and notes the request can cover all or a portion of records for each population).
The key takeaway is that if you are being sampled, IDOR often formalizes that fact through specific notices and requests.
Why sampling often inflates Illinois audit liability
Sampling can increase liability for three primary reasons:
1) Missing documentation turns “exempt” into taxable
Illinois audits commonly test exemption support. If you treat a sale as exempt but cannot produce support for the sampled transactions, auditors often treat the sale as taxable. Even if most of your exempt sales are valid, missing documentation inside the sample is what gets projected.
2) The sample period is not representative
If the sample month includes unusual promotions, staffing disruptions, POS (Point of Sale) changes, seasonality, supply chain changes, or a one-time contract, the “error rate” may reflect anomalies, not normal operations. When projected across years, it becomes an exaggerated exposure.
3) The sample plan can exclude context that explains differences
Many audit issues relate to reconciliation: deposits vs sales, tips, gift cards, returns, chargebacks, non-sales deposits, or intercompany transfers. If the sample does not capture the detail needed to prove those differences, the auditor may treat the variance as taxable sales.
IDOR’s audit information letter underscores that audits can include testing and review of source documents and general ledger accounts, which is why reconciliation and supporting records are central to audit outcomes.
Sampling disputes are easiest to address when caught early. You can create a free account with Sales Tax Helper to review whether a sample period may be distorted and identify the documentation that may help limit projected liability before projections are finalized.
What a good sample plan should include
A sample plan is more defensible when it has:
- A clearly defined population that matches how your business operates
- A sample period that reflects normal business conditions
- Clear instructions for what transactions will be tested and why
- A method for handling missing documents (especially for exemptions and resale support)
- A path for correcting errors before the error rate is finalized
If the plan is vague or overly broad, it can produce inflated projections.
How to challenge a distorted sample plan
Challenging a sample plan is not based on general objections such as “this is unfair.” It involves presenting a better method and supporting it with clear documentation.
Common defensible approaches include:
Request a Different Test Period
If the period includes abnormal events, you may propose an alternative period. The goal is not to delay the audit process, but to ensure the sample is representative.
Rebuild Documentation Before the Error Rate Is Finalized
If missing exemption or resale support is driving error rate, your best approach is to gather, validate, and organize documentation quickly, then submit it in a format the auditor can match the sampled transactions.
Provide a Reconciliation Narrative That Explains Variances
If deposits exceed reported sales due to non-taxable items, support this with documentation and a clear reconciliation schedule. The auditor needs a consistent and repeatable explanation.
Decline the Proposed Plan and Offer a Revised Approach
IDOR’s audit manual notes that you may accept or decline the proposed sample plan outlined in the Audit Proposed Sample Report. Declining does not mean refusing to cooperate; it signals that the plan should be revised to produce a more accurate result.
How to Protect Yourself
You can create a free account with Sales Tax Helper to review the key issues that typically arise when sampling and extrapolation are used in an audit. These often include:
- Identify the population and highest-risk categories. Especially exemptions, resale, and purchases with potential use tax exposure.
- Evaluate representativeness. Seasonality, abnormal months, POS conversions, promotions, staffing issues, or record gaps.
- Build a sample defense file. Index documents so that each sample transaction can be supported quickly and consistently.
- Prepare your “variance story.” POS vs GL (General Ledger) vs deposits, refunds, tips, gift cards, and non-sales deposits.
- Plan your response to IDOR sample letters. If you receive a Notice of Audit Population Sample Plan or a Proposed Sample Report, respond strategically and on time.
FAQ
Does IDOR always use sampling in an Illinois sales tax audit?
Not always. IDOR may review records in detail or in samples depending on the business and the audit approach. IDOR explicitly states that it may require records in samples or in detail.
How will I know if IDOR plans to sample?
IDOR’s audit manual describes a Notice of Audit Population Sample Plan that informs taxpayers of the intent to use a sample and provides information about the methodology. In some cases, you may also receive a proposed sample plan report requesting your agreement.
Can I disagree with IDOR’s sample plan?
Yes, and you should do so strategically when the plan is distorted. IDOR’s manual notes that the proposed sample report requests the taxpayer to indicate agreement and that the taxpayer may accept or decline the plan.
What is the biggest risk in sampling?
Projection. A small number of unsupported transactions in a sample can become a large assessment when projected across the audit period. The biggest drivers are missing exemption support, non-representative periods, and unresolved reconciliation variances.
What if I cannot produce records for the sample?
This presents a high-risk situation. IDOR’s audit information guidance explains that you are required to maintain records supporting items reported on your returns and warns that failure to produce requested records may result in penalties of up to $3,000 per filing period, subject to statutory and administrative provisions.
If records are missing, your strategy should focus on reconstruction, alternative documentation, and narrowing the population.
If sampling creates a large assessment, what dispute options exist?
IDOR’s audit information letter outlines post-audit resolution options and protest paths, including administrative hearings and the Illinois Independent Tax Tribunal for qualifying matters, plus Fast Track Resolution and Informal Conference Board review depending on posture.
Next Steps
If IDOR is using sampling in your Illinois sales tax audit, waiting until the projection is finalized can significantly limit available options. You can create a free account with Sales Tax Helper to review the sample plan, evaluate test period representativeness, exemption documentation, and reconciliation issues before an error rate becomes a multi-year assessment.