Wal-Mart Stores Inc. charged the wrong price to shoppers in California and the Midwest at a rate that exceeds those set by federal guidelines, according to two union-commissioned university studies released Monday.

The two studies said random purchases at 60 Wal-Mart stores in California found that the wrong price – sometimes higher, sometimes lower – came up 8.3 percent of the time; at 78 stores in Illinois, Indiana and Michigan, check-out scanners rang up the wrong price 6.4 percent of the time.

The National Institute for Standards and Technology says that for every 100 items scanned, no more than two should have the wrong price. The NIST’s last industrywide study, in 1998, found the rate at 3.35 per 100.

The recent studies were commissioned by the United Food and Commercial Workers, which has been unsuccessful in its attempts to organize Wal-Mart workers. The research was conducted by the University of Illinois-Chicago Center for Urban Economic Development and the University of California-Berkeley.