Wal-Mart Stores Inc will no longer keep a tight lid on its U.S. sales data after agreeing to share information with outside research firms to help it better understand how and why customers make choices.

The world's largest retailer said on Thursday it would provide sales data from its U.S. Walmart chain to Nielsen Holdings NV , bolstering an industry benchmark that has lacked Walmart's sales data for more than a decade.

Walmart has posted two straight years of declining sales at stores open at least a year, an industry metric known as same-store sales, as rivals chip away at its dominant market position. It aims to return to growth sometime this year.

Wal-Mart's Sam's Club warehouse club chain was already sharing data with outside firms.

SymphonyIRI said it would soon announce a similar pact with Walmart, which accounts for close to 11 percent of total U.S. retail sales. Wal-Mart will use research from the firms to better understand its customers.

Nielsen and SymphonyIRI gather and analyze sales data from U.S. chain stores ranging from grocery stores such as Kroger Co to mass merchants such as Target Corp to give manufacturers, Wall Street analysts and others insight into how various products are selling.

Without Walmart, the widely used data did not include the country's largest retailer. Walmart rang up $260.3 billion in sales in its latest fiscal year. U.S. retail sales topped $2.37 trillion in 2010, according to the National Retail Federation.

BIG DEAL FOR SUPPLIERS

The change is important for companies such as chocolate manufacturer Hershey Co and household products maker Clorox Co , which depend on Wal-Mart for a quarter or more of their sales.

Companies that sell their goods to Walmart were already able to see some performance data, but generally could not see how their sales stacked up against competitors.

This is a very big deal, to know how these companies' products are selling at Walmart, said Dave Kolpak, an analyst at Victory Capital Management.

Unlike many other chains, Walmart does not have a loyalty card, which can help retailers track how shoppers spend their money, what times of day they buy, how much they are buying on sale or with coupons, and other data.

Wal-Mart appears ready to figure out the best way to use its data and work with outsiders to gain more insights.

In February, it named Sam's Club executive Cindy Davis as the executive vice president of global customer insights, a new team that is studying customer trends.

Along with sharing its data, Walmart will also use Nielsen as its primary provider for market information, tools and training.

SymphonyIRI said it was working on a similar data agreement and that it would become Walmart's preferred provider of shopper insights, an area that Walmart is working on.

Now we just need them to start giving monthly same-store sales again, said Kolpak. Wal-Mart discontinued issuing monthly updates on its sales performance in 2009.

(Reporting by Jessica Wohl, additional reporting by Brad Dorfman, editing by Maureen Bavdek and Ted Kerr)

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