Gauging retail success or failure for the holidays is an increasingly difficult, and often unreliable, endeavor.

Online gift shopping and a penchant for gift cards have made the holiday season easier for consumers but harder on analysts whose job it is to predict holiday sales trends.

Current retail measurements are outdated, experts say, and investors now need to take a varied approach to making informed decisions about which retailers will see the fattest profits.

What was once a more cut-and-dried formula has in recent years become far more complicated, as consumers gravitate online and buy gift cards, and traditional shopping days lose their importance.

Same-store, or comp, sales, which have been a heavily relied upon statistic, measure sales at stores open at least a year. But neither online sales nor the bulk of holiday gift cards shows up in same-store sales data for November and December.

The retail barometer for the holidays has changed, said Martin Pyykkonen, an analyst with Global Crown Capital, noting that surveys that extrapolate the health of the season based on sales the day after Thanksgiving don't take into account the ease with which shoppers can shop later and later.

As Sucharita Mulpuru, multichannel retail analyst at Forrester, sees it, today's retail measuring sticks don't always consider the whole picture, and the whole picture is a pretty complex picture.

In general, retail metrics are pretty outdated and comp sales are one of the most outdated, Mulpuru said. Stores are absolutely being cannibalized by the Web. It's really a naive and outdated metric that has outlived its use.

The stakes are high in accurately predicting U.S. retail health during the holidays, which can contribute as much as 40 percent of yearly profits for retailers. That is especially true this year, when fears over consumer spending are rife amid high gasoline prices, a housing slowdown and a credit crunch.

Whether an outlook is rosy, bleak or somewhere in-between has major implications for retailers planning inventory, marketing budgets and promotions -- let alone for investors trying to guess the retail winners each year.

Companies from retail bellwether Wal-Mart Stores Inc to smaller chains like Tween Brands Inc have warned of challenges in the second half of their fiscal years, which generally end around January 31.

The National Retail Federation (NRF) has predicted 4 percent sales growth to $474.5 billion for the November and December holiday sales period, marking what could be the slowest gain in five years.

But that projection does not include January, when most gift cards are redeemed and counted towards retailers' sales. Those cards' sales can turn a month known for discounting into one where consumers make more full-price purchases, boosting profits.

The NRF's Scott Krugman calls this phenomenon the holiday shift, adding: It takes more patience to wait for January numbers to come in to see how the season went.

It's even harder to gauge the influence of the Internet on holiday retail sales. Still, experts agree that Web sales make up just a fraction of the overall retail pie, or less than 6 percent, according to Forrester.

While the Web may cannibalize store sales, it can also encourage browsing and send more shoppers back to stores, boosting pre-holiday sales.

Krugman said he foresees retail measurements eventually catching up to the increasing influence of the Internet. By the time it becomes a big enough chunk (of total retail sales), I feel confident that these measuring tools will be more in line with our needs, he said.

In the meantime, experts say investors need to rely on a broad array of data to make informed decisions.

U.S. Commerce Department data are crucial for a big-picture analysis, Krugman said, while Forrester's Mulpuru said there is value in consumer surveys that point to the popularity of certain categories, such as electronics or apparel.

I do think it's a combination of all of these data points, said Mulpuru, adding: You can't ignore the age-old tactics some of the equity analysts do -- they go to stores.

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