While shoppers lolled on the beach or camped under the stars this summer, it was holiday time for more than a few online retailers.

The shopping sites were running simulations to see what would happen over the holidays if customers showed up at their Web doors in numbers far beyond their wildest dreams -- or nightmares.

"Most sites don't make it through the test," said Ben Rushlo, senior manager of competitive research at online measurement firm Keynote Systems in San Mateo, Calif.

Better a practice crash on a slow day than a real one the day after Thanksgiving, something that happened to Wal-Mart last year.

"We saw lots of sites last year where they got more traffic than they could handle," said Mr. Rushlo.

Online shopping is projected to grow from $220 billion last year to about $259 billion this year. Over the holiday season, a survey for the National Retail Federation found consumers plan, on average, to do 30.2 percent of their shopping online, compared with 28.9 percent last year.

The reliability and sophistication of shops on the Web have improved in recent years, but there are still problems. A Shop.org survey of 150 retailers conducted by Forrester Research Inc. found half of retailers' Web sites are available most of the time -- but not all -- and fewer than half use performance tools to understand what their customers experience as they shop.

That's not good enough for customers who have come to see online stores as part of their shopping routine. Too many slow loading pages and cumbersome checkouts, and pretty soon they're clicking on a competitor's site -- and they don't even have to use gas getting there.

Many retailers have been scrambling to smooth the way through their Internet stores, trying to avoid bad designs and foul-ups that can result in a pile of half-full electronic shopping carts abandoned in disgust. The Forrester survey found 53 percent planning to improve checkout procedures and that 33 percent of the participating retailers were planning to invest more in live chat services that give consumers a way to ask questions from a real person. Some are even interested in proactive technology that would let a retailer send questions to consumers who appear to be stalled out.

That might be a little too much for some shoppers, said Jeanette Thomas, president of Web design and marketing firm Tachyon Solutions in Sewickley. Even if people are aware that retailers can watch them click their way through the store, it might feel a little Big Brotherish to be approached during a shopping session.

Still, consumers get frustrated if help isn't available when they need it.

David Flinn, director of professional services for Web performance firm Gomez in Lexington, Mass., recently sent an e-mail inquiry about a problem to a major retailer. The problem has since been resolved. Yet no one has responded to the e-mail. "It's not so much technology as retail practices," he said.

Following customers around to see what they do has never been easier than in the online world, which would make it seem easy to build the perfect site. But it turns out just following the clicks misses a lot of what may be happening on the other side of the computer screen.

That has bolstered the market for usability studies, whether through panels of computer users asked to perform certain tasks or through programs that track actual users. Keynote Systems has a tool that attempts to get busy customers to participate in a research session by sending out colorful pop-ups, such as a quacking duck for Google and a big red bulls-eye for Target.

To try to improve the customer experience, retailers are sending post-purchase e-mails, testing different versions of the same page and -- in one of the more visible moves -- allowing customers to talk amongst themselves by posting reviews of products. "I like to see what other people think about that product," said Ms. Thomas.

Home Depot Canada has created an Answer Depot where shoppers can ask questions and offer suggestions. Talbots, which has had live chat since the late 1990s, more recently added a way to reserve products in a nearby store, complete with a confirmation phone call from the store. Amazon.com recently launched a new "customer knowledge center" to help shoppers better research products such as high-definition TVs and game systems.

Meanwhile, there's little room for error in such basics as having the checkout working. Both Keynote Systems and Gomez regularly track the performance of major retail Web sites down to the tenth of a second. The tracking companies use tens of thousands of computers set up in various geographic locations to test the sites.

It may not look like a big deal if the same task takes 2.26 seconds to perform on Wal-Mart's site and 6.62 seconds on Comp USA's site, as a Gomez response time rating found in September. But every second spent waiting adds up. The test computers may even underestimate the average customer's experience. "What we've found is, from the home, it's almost three times slower," said Mr. Flinn.

Extra loads around the holiday can exacerbate the problem, as can colorful images of sweaters in four different shades and extras such as video clips and music.

Then, if the marketing team comes up with a great deal offering, say, 200 free iPods, one with every $500 purchase over two days in December, that can send even a solid Web site over the edge, said Mr. Flinn. "That's going to generate more traffic than they may have expected."

Web store operators have learned to have backup plans. If off-peak simulations show they need more computer capacity to handle a rush, many work out deals with outside vendors who rent cyberspace on an as-needed basis, said Ms. Thomas.

In a sign of how much this season matters, clients of Gomez have asked the company not to tinker with its measurement network of computers until the Christmas shopping season is over. They don't want any false alarms.

For those who didn't already run tests to help brace for the best/worst-case scenarios, Mr. Rushlo said there may be only one thing to do. "Grab your rosary beads and hope the site doesn't crash."