SPRING VALLEY, Calif. — Since the Fresh & Easy grocery chain was founded five years ago, it has opened 150 stores in California and positioned itself as a hip, socially responsible company.

A cross between Whole Foods and Trader Joe’s, the company brags that its house brands have no artificial colors or trans fats, that two-thirds of its produce is grown locally and that its main distribution center is powered by a $13 million solar installation.

But in one crucial respect, Fresh & Easy is just like the vast majority of large American retailers: most employees work part-time, with its stores changing many of their workers’ schedules week to week.

At its store here, just east of San Diego, Shannon Hardin oversees seven self-checkout stations, usually by herself. Typically working shifts of five or six hours, she hops between stations — bagging groceries, approving alcohol purchases, explaining the checkout system to shoppers and urging customers to join the retailer’s loyalty program, all while watching for shoplifters.

“I like it. I’m a people person,” said Ms. Hardin, 50, who used to work as an office assistant at a construction company until times went bad.

But after nearly five years at Fresh & Easy, she remains a part-time worker despite her desire to work full-time. In fact, all 22 employees at her store are part-time except for the five managers.

She earns $10.90 an hour, and with workweeks averaging 28 hours, her yearly pay equals $16,500. “I can’t live on this,” said Ms. Hardin, who is single. “It’s almost impossible.”

While there have always been part-time workers, especially at restaurants and retailers, employers today rely on them far more than before as they seek to cut costs and align staffing to customer traffic. This trend has frustrated millions of Americans who want to work full-time, reducing their pay and benefits.

“Over the past two decades, many major retailers went from a quotient of 70 to 80 percent full-time to at least 70 percent part-time across the industry,” said Burt P. Flickinger III, managing director of the Strategic Resource Group, a retail consulting firm.

No one has collected detailed data on part-time workers at the nation’s major retailers. However, the Bureau of Labor Statistics has found that the retail and wholesale sector, with a total of 18.6 million jobs, has cut a million full-time jobs since 2006, while adding more than 500,000 part-time jobs.

Technology is speeding this transformation. In the past, part-timers might work the same schedule of four- or five-hour shifts every week. But workers’ schedules have become far less predictable and stable. Many retailers now use sophisticated software that tracks the flow of customers, allowing managers to assign just enough employees to handle the anticipated demand.

“Many employers now schedule shifts as short as two or three hours, while historically they may have scheduled eight-hour shifts,” said David Ossip, founder of Dayforce, a producer of scheduling software used by chains like Aéropostale and Pier One Imports.

Some employers even ask workers to come in at the last minute, and the workers risk losing their jobs or being assigned fewer hours in the future if they are unavailable.

The widening use of part-timers has been a bane to many workers, pushing many into poverty and forcing some onto food stamps and Medicaid. And with work schedules that change week to week, workers can find it hard to arrange child care, attend college or hold a second job, according to interviews with more than 40 part-time workers.

To be sure, many people prefer to work part time — for instance, college students eager for extra spending money and older people earning money for presents during the holiday season.

But in two leading industries — retailing and hospitality — the number of part-timers who would prefer to work full-time has jumped to 3.1 million, or two-and-a-half times the 2006 level, according to the Bureau of Labor Statistics. In retailing alone, nearly 30 percent of part-timers want full-time jobs, up from 10.6 percent in 2006. The agency found that in the retail and wholesale sector, which includes hundreds of thousands of small stores that rely heavily on full-time workers, about 3 in 10 employees work part-time.

Retailers and restaurants use so many part-timers not only because it gives them more flexibility, but because it significantly cuts payroll costs.

According to the Bureau of Labor Statistics, part-time workers in service jobs received average compensation of $10.92 an hour in June, which includes $8.90 in wages plus benefits of $2.02. Full-time workers in that sector averaged 57 percent more in total compensation — $17.18 an hour, made up of $12.25 in wages and $4.93 in benefits. Benefit costs are far lower for part-timers because, for example, just 21 percent of them are in employer-backed retirement plans, compared with 65 percent of full-timers.

At the Fresh & Easy store here, Ms. Hardin is forever urging her boss to give her more hours, she said, but instead, “they turn around and hire more people.” Some weeks, her boss gives her an extra shift when a co-worker is sick or on vacation.

Officials of Fresh & Easy, which is owned by Tesco, the largest supermarket company in Britain, declined to be interviewed. But the company noted that its entry-level pay was $10 an hour, substantially higher than at most retailers, with quarterly bonuses on top of that. Also, the company said it offered excellent benefits, including health insurance to anyone averaging more than 20 hours a week.

Ms. Hardin said her recent quarterly bonuses averaged less than $200, and while she appreciated the health insurance, she often could not afford the co-pays to see a doctor.

To supplement her income, she moonlights 15 or so weekends a year as a security guard at San Diego Chargers and San Diego State football games. But she still has such a hard time making ends meet, she said, that she has gone to the movies just three times in the last five years. Nor does she own a television.

“A couple of people offered me a used TV, but I can’t afford cable,” she said. “I have a tooth that’s falling apart, but I can’t afford the crown for it.”

Juggling Schedules

At the Jamba Juice shop at 53rd Street and Lexington Avenue in Manhattan, along with the juice oranges and whirring blenders is another tool vital to the business: the Weather Channel.

The shop’s managers frequently look at the channel’s Web site and plug the temperature and rain forecast into the software they use to schedule employees.

“Weather has a big effect on our business,” said Nicole Rosser, Jamba’s New York district manager.

If the mercury is going to hit 95 the next day, for instance, the software will suggest scheduling more employees based on the historic increase in store traffic in hot weather. At the 53rd Street store, Ms. Rosser said, that can mean seven employees on the busy 11-to-2 shift, rather than the typical four or five.

Such powerful scheduling software, developed by companies like Dayforce and Kronos over the last decade, has been widely adopted by retail and restaurant chains. The Kronos program that Jamba bought in 2009 breaks down schedules into 15-minute increments. So if the lunchtime rush at a particular shop slows down at 1:45, the software may suggest cutting 15 minutes from the shift of an employee normally scheduled from 9 a.m. to 2 p.m.

Karen Luey, Jamba’s chief financial officer, said the scheduling software “helped us take 400, 500 basis points out of our labor costs,” or 4 to 5 percentage points, a savings of millions of dollars a year.

At Jamba Juice, which has 770 outlets, managers used to piece together their stores’ weekly schedules on an Excel spreadsheet. It took managers about two hours to slot in 25 to 30 employees, all generally part-time except for the store manager and one or two shift managers. With the Kronos software, scheduling takes just 30 minutes.

The software keeps tabs on when workers are available, their skills and who makes the most sales per hour. While such software is a powerful tool, management’s judgment is still important, said Aron J. Ain, Kronos’s chief executive. “The budget is how many people you need at a certain time,” he said, “but the magic is deciding who is to work at what time.”

The rise of big-box retailers like Walmart, with their long operating hours and complex staffing needs, has contributed to the increase in part-timers.

Mr. Flickinger, the retail consultant, said when Walmart spread nationwide and opened hundreds of 24-hour stores in the 1990s, that created intense competitive pressures and prompted many retailers to copy the company’s cost-cutting practices, including its heavy reliance on part-timers.

Susan J. Lambert, an expert on part-time work and a professor of organizational theory at the University of Chicago, said the use of part-timers had also escalated because of the declining power of labor unions. “They set a standard for what a real job was — Monday through Friday with full-time hours,” she said. “We’ve moved away from that.”

Many corporations place store or restaurant managers under strict limits about what their payroll or employee hours can be each week, usually based on a formula tied to sales. These formulas usually give managers little flexibility to increase the hours assigned.

David Henson, a former assistant manager at a Walmart in Thief River Falls, Minn., said part-timers would sometimes come into his office on the brink of tears.

“A lot of them were single mothers. They said they weren’t earning enough to support their families,” he said. “They desperately wanted more hours, but we weren’t able to give them.”

Some, Mr. Henson said, were eager to take second jobs. But if they said they were unavailable during certain hours, then the managers and scheduling software would reduce their hours further, he said. Many workers concluded that it was simply not worth it.

David Tovar, a Walmart spokesman, said that less than half of Walmart’s hourly employees were part-time and that the company provided better wages and benefits than many competitors. But he acknowledged that part-time employees with less availability were typically assigned fewer hours.

Katherine Lugar, executive vice president of the Retail Industry Leaders Association, said that the industry’s scheduling practices worked well, and that retailers did their best to accommodate employee needs. “Happy employees provide better service,” she said.

She noted that millions of Americans preferred part-time work. “Many individuals come to retail because it is flexible, like the working mom who wants to work when kids are in school, or the graduate student,” she said.

When the Hours Fade

The day after Desmond Anthony graduated from Western Carolina University, he moved to Manhattan with the dream of becoming a Broadway actor and singer.

He knew he had to support himself with something else, and by Week 2, he had applied for 20 retail jobs, including one at the sprawling Express store in Herald Square, an emporium of slim jeans, sequined T-shirts and booming music.

“When I first walked into Express, I said, ‘Oh my God, this place is awesome and there’s music and it looks like a happening place,’ ” Mr. Anthony said.

Express offered him a job the next day. Mr. Anthony, 6-foot-4 and with a booming voice and big smile, said that after receiving just four hours of training, he began alternating as a greeter, cashier and sales floor assistant.

At first, he usually worked five days a week, often racking up 30 hours. But after several months, he said, he and many co-workers had their weekly hours cut to 12 or 15 and occasionally none at all.

“I’d go to the managers and say, ‘What is the issue? Am I not pulling my weight?’ ” he said. “And they’d say, ‘We just don’t have enough money.’ ”

“ ‘So how am I supposed to support myself? ’ I asked, and they said that was not their problem.”

Mr. Anthony said it was hard to survive. At $8.25 an hour, 15 hours a week equaled about $500 a month. His share of the monthly rent was $800, with several hundred more for utilities, phone and subway fares. Some days he went hungry, he acknowledged, and he repeatedly turned to his parents for help.

He and his co-workers held out hope that, come the holiday season, their hours would pick up. “But then they hired 15 more workers,” he said.

The store’s schedule for each coming week, he said, was supposed to be posted on Wednesdays, but often didn’t go up until Friday or Saturday. With so little notice, he sometimes had to scrap plans for auditions.

At one point, he said, his weekly schedule dwindled to two assigned days and two or three days when he was supposed to call the store in the morning to see whether managers wanted him to come in that day.

Mr. Anthony quit last February, upset that Express had given him an annual raise of just 25 cents an hour. He now works at a Zara apparel store on Fifth Avenue, which, he said, gives him 30 hours a week and does more to accommodate his scheduling needs.

Express says that about 85 percent of its employees are part-time. “It’s really more for flexibility than for anything else,” said Michael Keane, the company’s executive vice president for human resources. “It helps our ability to match associate staffing to traffic levels.”

Mr. Keane said many young people were eager to work part-time there, attracted by a hip atmosphere and the clothing discounts for employees.

With regard to Mr. Anthony’s complaints, Barbara Coleman, an Express spokeswoman, said stores aimed to post worker schedules a week or two in advance. “An associate will be notified in advance if they are scheduled for a call-in shift,” she said.

As for the hiring surge that upset Mr. Anthony, Ms. Coleman said, as the holidays approach, Express typically increases its part-time work force by nearly 20 percent to accommodate extended hours and the rush of shoppers.

In New York’s fiercely competitive retail world, Mr. Anthony’s experience is not unusual. Workers at Abercrombie & Fitch, Nine West and Bed Bath & Beyond told similar stories.

A 2011 survey of 436 employees at retailers in New York City, as diverse as luxury establishments on Fifth Avenue and dollar stores in the Bronx, found that half of the city’s retail workers were part-time and only one in 10 part-time workers had a set schedule week to week. One-fifth said they always or often had to be available for call-in shifts, according to the survey, which was overseen by researchers at City University of New York.

“We’re seeing more and more that the burden of market fluctuations is being shifted onto the workers, as opposed to the companies absorbing it themselves,” said Carrie Gleason, executive director of the Retail Action Project, an advocate for retail workers that helped conduct the survey and is financed by foundations and the Retail, Wholesale and Department Store Union.

That union wants more labor deals like the one it has at Macy’s flagship store in Herald Square in Manhattan. Although that store has many part-timers, the more senior workers can reserve days off and learn their schedule six months in advance.

Mr. Flickinger, the retail consultant, said companies benefited from using many part-timers. “It’s almost like sharecropping — if you have a lot of farmers with small plots of land, they work very hard to produce in that limited amount of land,” he said. “Many part-time workers feel a real competition to work hard during their limited hours because they want to impress managers to give them more hours.”

Ms. Rosser, the Jamba Juice district manager, amplified on the advantages.

“You don’t want to work your team members for eight-hour shifts,” she said. “By the time they get to the second half of their shift, they don’t have the same energy and enthusiasm. We like to schedule people around four- to five-hour shifts so you can get the best out of them during that time.”