It has specific guidelines for deciding who gets called in. Understandably, however, the agency isn't about to tell outsiders what those guidelines are. That said, CPAs have a general idea how the guidelines work. Independent contractors' returns fall outside the normal range more often than those of W-2 wage earners because contractors claim more deductions. The typical business return from an independent contractor may claim 30 different deductions.
Contractors also claim the expenses that come from running their own business.
Normal business expenses become audit-worthy when they reach abnormal levels. The muckety muck statisticians in Washington, D.C., know exactly how much you should be spending. If you're a dentist, they know how much you should be deducting for advertising, because they know how much every dentist in America spends on advertising. You'll get an invitation to stop in for a visit. They're what I call insults to the IRS' intelligence.