Check tampering, bogus billing and other schemes cost U.S. businesses up to seven percent of revenue every year, with smaller firms  hit hardest, according to the Association of Certified Fraud Examiners.

In a report released this week, the Austin, Tex.-based trade group said businesses with fewer than 100 employees lose a median of $200,000 a year to fraud, higher than losses at larger employers.

The findings are based on more than 900 cases of occupational fraud investigated by the group over the past two years.

The report identified check tampering as the most common form of fraud, accounting for a fourth of all small-business cases.

The group said check tampering occurs most often at businesses where various cash disbursement duties aren't separated.

Among other fraud prevention tips, it suggests small-business owners adopt a code of ethics  for all employees, conduct thorough background checks on new recruits, and perform regular audits.