Households making less than $25,000 a year are audited by the IRS at a rate of 13 per 1,000 returns. Households making more are audited at a rate of 2.6 per 1,000 returns. These figures come from researchers at Syracuse University, who data-mined public IRS records.
From The Los Angeles Times:
It's impossible to overstate how unjust and wasteful this is. These families tend to be our most financially vulnerable, struggling to pay for basic expenses. They're the least likely to have professional tax advice, and most likely to have limited education or for English not to be their first language.
When they receive an audit letter from the IRS, their only option to obtain more information is by telephone. But as the agency's national taxpayer advocate, Erin M. Collins, observed in her most recent report, last year only 11% of callers to the IRS help line got through to a customer service representative.
"Among the lucky one in nine callers who was able to reach a CSR," Collins wrote, "hold times averaged 23 minutes." But that was an average; many callers spent much longer on hold.