Flex time will take on a different meaning at Wal-Mart stores this year.

The world’s largest retailer plans to use advanced software to better match foot traffic with staffing. Full-time and part-time cashiers tell the company what hours they prefer to work, as well as what hours they are available. A computer system devises a schedule three weeks ahead of time to match the number of available cashiers with the right time slots to get customers out the door rapidly.

The system will give the company flexibility to place more workers on the floor in the early evening rather than during the slow morning hours. It is designed to boost sales and customer satisfaction, but it could prove difficult for employees seeking to balance work and family life, or to estimate and match their weekly earnings with their bills.

The company plans to roll out the system by year’s end throughout the nation.

“This system affords us far more predictability than what we had before,” said John Simley, a spokesman for the Bentonville, Ark., company. “The goal is to improve the shopping experience for the customer and improve the work environment for the associate.”

The goal also is to crank out more sales during busy periods. Other chains also are trying to make it easier for customers to spend more money on more merchandise without worrying about the hassle of getting through checkout.

“This is the first time anyone has done something like this on so large a scale,” said Joseph Beaulieu, an analyst for Morningstar in Chicago. “This is probably a long-term trend.”

At the Wal-Mart in Secaucus, N.J., manager Tracy Ferschweiler was busy recently planning how many cashiers he will need the week of Jan. 20. His 80 cashiers notified him of the hours they want and the hours they could work if needed. That information is fed into a computer along with his sales forecast for that week. The result should mean shorter customer lines.

Wal-Mart already is known for its sophisticated use of data systems to track inventory and notify vendors to deliver goods on a just-in-time basis. Other retail chains also are believed to be experimenting with advanced work scheduling systems, including Payless ShoeSource and RadioShack.

Big box retailers such as Lowe’s, Target and Home Depot already have implemented flexible staffing systems, according to Stephanie Hoff, retail analyst for Edward Jones.

“All retailers are trying to do this on a per-store basis because local tastes differ from store to store, and a one-size-fits-all approach doesn’t work,” she said.

For example, the Wal-Mart in Secaucus gets busy from 5 p.m. until closing at 10, according to Ferschweiler, the store manager. However, a Wal-Mart in Hornell, N.Y., where there is a smaller customer base, slows dramatically after 7 p.m.

Critics said Wal-Mart is merely trying to avoid paying overtime and benefits to some workers. They say the system will push out senior employees who can’t adapt to the flexible schedule.

“It is part of a deliberate strategy to force out many of the full-time employees … who are used to certain schedules and will have to leave the company or go part-time because they won’t be able to meet the new criteria,” said Nu Wexler, a spokesman for Wal-Mart Watch, a labor-affiliated group.

Wal-Mart spokesman Simley said the company isn’t trying to limit worker hours and that it provides benefits for both full-time and part-time associates.

Wal-Mart has had a sometimes contentious relationship with its 1.3 million workers. Employees in Pennsylvania won a $78.5 million judgment against the company for doing unpaid work and working through their rest breaks. Wal-Mart is appealing a $172 million unpaid work dispute in California, and it reached a $50 million settlement in a similar lawsuit filed in Colorado.

The company earned $11.2 billion last year on $312 billion in sales. Last week, Wal-Mart posted its weakest December sales results in six years. The retailer reported strong sales of flat-panel TVs, but it has struggled to attract higher-income shoppers with better apparel and home furnishings.

Wal-Mart needs to be more productive and provide better service without hiring more workers, according to Walter Loeb, president of Loeb Associates, a New York market research firm.

“They want more people in the store when customers are there, not on a 9-to-5 basis,” Loeb said. “The danger is that they could lose a lot of associates who don’t want to work certain hours.”

Wal-Mart has been experimenting with a pilot scheduling program in 39 states for the past few months, Simley said.

Latoya Machado, a cashier in Grapevine, Texas, near Dallas, said her previous work schedule changed every week. Now, she works Monday through Thursday between the hours of 5 a.m. and 5 p.m. and on Sunday between 3 and 9 p.m.

It doesn’t bother her that she no longer gets overtime because she would rather be certain she can leave by 5 p.m. each day to pick up her two sons from day care. Previously, she had to arrange for someone else to pick them up or pay a late fee.

Machado said the Texas store has more associates available at 5 p.m. when it gets busy.

“There are less lines for the customers, and I don’t have to pick up anybody else’s slack,” she said.