Audit selection feels random from the outside. It is not. The overwhelming majority of examinations start with arithmetic: a scoring model flags a return as statistically unusual, a computer matches documents and finds a gap, or a related case pulls yours in. Understanding the actual triggers replaces superstition with risk management.

The Scoring Models

Every filed return receives a DIF score - a statistical measure of how far its deductions, credits, and ratios deviate from norms for similar incomes and industries. High scores get human review; humans select audits. The practical implication: it is not the size of a deduction that flags you, it is the proportion. A $30,000 charitable deduction on $60,000 of income screams; the same deduction on $400,000 whispers. Schedule C filers live under the most scrutiny because the models know self-employment is where the noncompliance lives - particularly cash-heavy industries, perennial losses that look like hobbies, and vehicle or home-office claims at implausible percentages.

The Matching Machine

Separate from scoring, the automated underreporter program matches every W-2, 1099, K-1, and broker statement against what you reported. A missed 1099 does not require anyone to suspect you; the computer finds it and issues a CP2000 proposing tax on the difference. With brokers, exchanges, and payment platforms all reporting now, the matching net catches more every year - and the correct response to a CP2000 is a reconciliation, not a check, because the IRS's proposed figure routinely ignores basis and deductions you are entitled to.

The Human Triggers

Beyond the machines: round numbers everywhere on a return signal estimation; amended returns claiming large refunds get screened; your business partner's audit can become your audit through related-party selection; and whistleblower claims from ex-spouses and former employees start more exams than anyone admits.

None of this means forfeiting legitimate deductions - it means claiming them with documentation proportionate to their visibility. If your return carries known flag patterns, the time to organize the proof is before the letter, and if the letter already came, the response strategy is a conversation we should have this week.