How Does the IRS Choose Who to Audit and What Happens

The IRS chooses who to audit through five main channels: a computer scoring system that flags statistical outliers, automated matching of your return against W-2s and 1099s, tips from whistleblowers, spillover from audits of related businesses or partners, and a small random sample. Only about 0.4% of individual returns filed for tax year 2022 were examined, so most filers never hear from an auditor at all.¹ But that headline number hides sharp variation by income and by how you earn it, and the selection process itself is more mechanical than most people assume.

The Computer Score Every Return Gets

Every individual return receives a numerical score through the Discriminant Function system, known as DIF. The formula compares each line of your return against statistical norms for taxpayers with similar income, filing status, and occupation. Returns that deviate sharply score higher, signaling greater potential for a tax change if examined.² A separate Unreported Income DIF, or UDIF, estimates the likelihood that income was left off entirely — unreported stock sales, business receipts, or dividends.³

The exact formulas are classified. The IRS has never published the weights it assigns to particular deductions or income ratios, and the formulas are updated periodically using data from random compliance audits. What is public is the general process: each year the IRS sets a national cutoff score for each return category, and only returns above that floor get flagged for human review.²

A high DIF score doesn’t automatically trigger an audit. Once a return clears the cutoff, it lands on a classification examiner’s desk. That person reviews it manually and decides whether the flagged issues are worth pursuing or whether the return should be accepted as filed. The algorithm catches statistical outliers, not necessarily errors, so a taxpayer who legitimately had an unusual year might score high without owing another dime.

Document Matching Against W-2s and 1099s

The IRS receives roughly two billion information returns each year — W-2s from employers, 1099s from banks and brokerages, and similar documents from anyone who paid you money.⁴ An automated system compares those third-party numbers against what you reported. When it finds a mismatch, you’ll typically receive a CP2000 notice proposing changes to your return based on the discrepancy.⁵

The CP2000 isn’t technically an audit. It’s an automated adjustment proposal. But it functions like one for the person receiving it. Common triggers include forgetting to report a freelance 1099, missing interest from a savings account, or misreporting stock sale proceeds. You get 30 days to respond, or 60 days if you live outside the United States, either agreeing to the changes or documenting why the IRS figure is wrong.⁵

Cash and Payment App Reporting

Matching extends beyond wages and investment income. Any business receiving more than $10,000 in cash from a single transaction, or related transactions, must file Form 8300, which the IRS cross-references against the payer’s return.⁶ Pay $15,000 cash for a used car when your reported income can’t plausibly support that purchase, and the mismatch creates a flag.

Payment apps and online marketplaces report under Form 1099-K. Under changes enacted in the One, Big, Beautiful Bill, the reporting threshold reverted to $20,000 in gross payments across more than 200 transactions per year — the level that existed before the American Rescue Plan’s attempted reduction to $600.⁷ Sell goods or freelance through PayPal, Venmo, or Etsy above that threshold, and the platform reports your payments while the IRS matches them against your return.

Income Level and Self-Employment

Income is the single strongest predictor of audit risk. For tax year 2022, the IRS examined 4.0% of returns reporting $10 million or more in total positive income, compared with just 0.1% of returns in the $50,000 to $200,000 range.¹ The gap widens across the upper brackets:

$1 million to $5 million: 1.1% audit rate
$5 million to $10 million: 3.1% audit rate
$10 million or more: 4.0% audit rate

The agency has publicly committed to higher scrutiny of top earners while keeping audit rates for taxpayers earning under $400,000 at historically low levels, and its strategic plan targets significantly higher examination rates for individuals reporting eight figures of income by 2026.¹

Self-employment is the other major attention-getter. When you file a Schedule C, every dollar of income and every deduction is self-reported. No employer verifies your numbers. The IRS knows this creates opportunity for error and for cheating, so returns with large Schedule C deductions relative to income carry extra DIF weight. Patterns that consistently draw scrutiny include reporting business losses year after year, claiming vehicle expenses without mileage logs, mixing personal and business expenses, and taking deductions disproportionate to the revenue a business generates. Cash-intensive businesses like restaurants and salons face even more attention because the IRS has limited third-party records to verify their receipts.

Tips From Whistleblowers

The IRS accepts tips from people who have knowledge of tax fraud or significant underreporting. Anyone can submit information through Form 211, and if the tip leads to collected taxes, the informant may receive an award. When the disputed amount exceeds $2 million, and the taxpayer’s gross income exceeds $200,000 in at least one relevant year, the whistleblower qualifies for a mandatory award of 15% to 30% of collected proceeds. Smaller cases can still receive discretionary awards.⁸

The IRS doesn’t just file these tips away. Credible, specific information about unreported income, abusive tax shelters, offshore evasion, or money laundering can directly prompt an examination. A disgruntled former business partner or ex-spouse with access to financial records is one of the more common sources. Because the financial incentive is real, tips tend to be detailed and well-documented, which makes them particularly useful when the IRS decides where to focus resources.

Spillover From Related Audits

Audits can be contagious. When the IRS examines a partnership, corporation, or estate, it frequently pulls the returns of everyone connected to that entity. The logic is direct: if the partnership overstated a deduction, every partner who claimed a share of that deduction on a personal return carries the same error. The agency traces money through these relationships to check that income isn’t being shifted or hidden.

Partnerships face their own audit regime under rules established by the Bipartisan Budget Act. When a partnership return is selected, the IRS sends Letter 2205-D to the partnership itself, followed about 30 days later by a formal Notice of Administrative Proceeding.⁹ Adjustments made at the partnership level can flow down to individual partners, potentially triggering additional tax for people who had no idea the partnership was under examination. If you’re a partner in a business venture, the compliance of the partnership itself directly affects your own audit exposure.

Pure Random Selection

Some returns are picked through random sampling regardless of what they look like. The National Research Program selects returns to gather data on how accurately different types of taxpayers report income and deductions.¹⁰ The Taxpayer Advocate Service has described these taxpayers as “guinea pigs” who serve the system’s needs even when they’re fully compliant.¹¹

NRP audits tend to be more thorough than typical examinations. Because the goal is a complete statistical picture, examiners may request documentation for every line item on the return, not just the ones that look suspicious. The IRS uses this data to update its DIF scoring formulas, estimate the overall tax gap, and help Congress evaluate tax policy. The sample is small — historically around 13,000 to 14,000 individual returns per year — so any given taxpayer’s chance of random selection is very low.¹⁰

How Far Back the IRS Can Look

The IRS doesn’t have forever to select a return. The general rule gives the agency three years from the date you filed (or the due date, whichever is later) to assess additional tax.¹² That three-year window is the Assessment Statute Expiration Date, and once it runs, the IRS loses authority to adjust that year’s return.¹³

Three exceptions extend or eliminate that deadline:

Substantial omission gives the IRS six years instead of three if you left off more than 25% of the gross income you should have reported.¹²
Fraud has no limit. If you filed a false or fraudulent return with intent to evade tax, the IRS can audit that year decades later.¹³
Failure to file has no limit either. If you never filed, the three-year clock never starts, and the IRS can assess tax at any time.¹²

Filing a return, even one with mistakes, starts the clock. Not filing leaves you exposed indefinitely. Cross the 25% omission line and you’ve handed the IRS twice as long to find it.

1
Internal Revenue Service. IRS Data Book, 2024
2
Internal Revenue Service. 4.1.2 Workload Identification and Survey Procedures
3
Internal Revenue Service. Test of Unreported Income DIF Scores
4
Internal Revenue Service. About Form W-2, Wage and Tax Statement
5
Internal Revenue Service. Topic No. 652, Notice of Underreported Income – CP2000
6
Internal Revenue Service. Form 8300 and Reporting Cash Payments of Over $10,000
7
Internal Revenue Service. Treasury, IRS Issue Proposed Regulations Reflecting Changes From the One Big Beautiful Bill
8
Internal Revenue Service. Submit a Whistleblower Claim for Award
9
Internal Revenue Service. BBA Partnership Audit Process
10
Taxpayer Advocate Service. Legislative Recommendations – National Research Program Audits
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Taxpayer Advocate Service. Compensate Taxpayers for No Change National Research Program Audits
12
Office of the Law Revision Counsel. 26 USC 6501 – Limitations on Assessment and Collection
13
Internal Revenue Service. Time IRS Can Assess Tax