With its earnings and sales more or less known, the key thing to look for in Wal-Mart's ( WMT) earnings report on Tuesday is how the company performed on its expenses -- and what it expects for the rest of the year.

Those are, of course, key figures to examine in any company's earnings report. But with Wal-Mart representing more than 10% of U.S. retail sales, excluding sales of motor vehicles, trends at the retail behemoth could well indicate what investors should expect from the rest of the retail industry.

"The retail earnings season starts tomorrow," said Fran Radano, a research analyst for Gartmore Global Investments. "This sets the tone for the next three weeks."

Wal-Mart, which reports weekly and monthly sales figures, has already warned investors of below-plan sales. Benefiting from Easter falling in April this year rather than March last year, the company posted a comparable-store sales increase of 4.6% last month. For the last three months, the company's same-store sales, which compare results at outlets open more than one year, increased 2.5%, compared with a 7.8% increase in the same three months last year.

Despite the disappointing sales, the company reaffirmed its earnings guidance last week, saying that it expects its earnings to come in at the high end of its previously stated range of 40 cents to 42 cents a share.

For the 13-week period ended May 2, which roughly coincides with its first quarter, Wal-Mart posted $58.37 billion in retail sales. That amount does not include membership revenue from its wholesale chain Sam's Club or results from McLane, the grocery distributor that Wal-Mart recently agreed to sell to Berkshire Hathaway.

Analysts are expecting Wal-Mart to earn 42 cents a share on about $60.69 billion in sales, according to Thomson Financial/First Call. In the year ago period, Wal-Mart earned $1.65 billion, or 37 cents a share, on $55.42 billion in revenue.

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