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 studies released yesterday.

Attorneys general in Illinois and California said the reports raised serious concerns. Connecticut Attorney General Richard Blumenthal, on a conference call held by the studies' backers, said his office would investigate what he called a "culture" of inaccuracy.

The two studies said random purchases at 60 Wal-Mart stores in California found that the wrong price 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. In both states, some prices rang up higher and some were lower.

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 for years, and released by a UFCW-backed campaign group, Wake Up Wal-Mart. The research was conducted by the University of Illinois-Chicago Center for Urban Economic Development and the University of California-Berkeley.

"A majority of Wal-Mart stores tested in this evaluation of price accuracy demonstrated errors in pricing that exceeded federally accepted standards for large retail establishments," the California and Midwest studies concluded.

The researchers said the average cost of overcharges was more than that of undercharges.

Wal-Mart said its last internal audit found an error rate of 2.4 percent — less than the 1998 national study by NIST — and slammed the union studies as incomplete and outdated. The company noted that the studies' authors say their results are specific to those states and cannot be generalized for the entire country.

"This desperate attack has more holes than a pasta strainer. This is another paid attack by union critics," Wal-Mart spokeswoman Sarah Clark said.