The highest risk of audit is born by the US President who is automatically audited by the IRS every year. President Trump says this is very unfair.
Any taxpayer who omits an income item filed by a third party (a W2 form by an employer or a bank interest filing, for example) faces a high chance of correspondence audit. These are computer-generated exams based on mis-matching data. These are the most common audits.
High income earners and self-employed people who take unusual deductions are audited more frequently but audit rates may depend on the specific situation.
People who file tax returns showing zero income face a 1 in 20 risk of audit.
All other taxpayers face less than 1 in 100 audit risk.