Wal-Mart

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, the world's biggest retailer, said same-store sales rose 6.9% in August, well ahead of the company's own optimistic expectations for the month.

Late in August, the company projected that comparable sales would climb 4% to 6% for the month.

Total sales for the four-week period ended Aug. 29 rose 13.7% to $19.53 billion, up from $17.18 billion in the same period last year. The Wal-Mart division's sales reached $13.41 billion, up 13.8% from the prior year.

Sam's Club had sales of $2.65 billion last month, an 11.3% increase from a year ago. The international division's sales for the four-week period rose 15.2% to $3.48 billion.

For the September five-week period, Wal-Mart expects same-store sales growth in the 3% to 5% range.