When it comes to retailers, comparable-store sales are a good indicator of strength. Assuming the number is positive, it shows that the retailer is selling more goods than in the previous period at the same locations. For those who don't know, comp sales compare the sales (thus the name) of a company from one period to the next (usually year-to-year), including only those locations that reported sales in both periods. It's more useful than the overall sales number, which can seem strong simply because a retailer opened a plethora of new stores in the period.

What better place to look at sales data than discount retailers, whose main goals are low prices and large quantities? Overall, February was a good month for big-box retailers. Each of the three discounters posted decent comp sales growth, but Motley Fool Stock Advisor pick Costco (NASDAQ:COST) led the way with an 8% jump for the month -- more than twice that of its rivals. Target (NYSE:TGT) increased its comps by 3.6%, while Wal-Mart (NYSE:WMT) generated a 3.2% gain.

Target had the toughest road, since it had to improve upon 9% growth from last year. Costco's strong month is even more impressive when you consider that it followed last year's 7% increase. As the biggest of the three, Wal-Mart continues to maintain steady growth, having posted a 4.1% gain in comps last year.

As the table below shows, this marks the seventh consecutive month that Costco has led the way. It reported a 9% gain in January, while Target and Wal-Mart were also growing quickly, with 5.2% and 4.7% growth, respectively.

8/05

9/05

10/05

11/05

12/05

1/06

2/06

Costco

9%

11%

10%

6%

7%

9%

8%

Target

6.3%

5.6%

5.7%

2.6%

4.7%

5.2%

3.6%

Wal-Mart

3.3%

3.8%

4.3%

4.4%

2.2%

4.7%

3.2%


Looking ahead to next month, Target and Wal-Mart expect slower growth, with both predicting same-store sales increases of 1% to 3%. Each blames the Easter Bunny's late arrival this year.

I think that each of these companies will continue to perform well, and that each merits a closer look, particularly as economic conditions cause consumers to be more frugal. Based on comps data, however, Costco is clearly outperforming the competition.

For more on the big-box battles:

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Fool contributor Mike Cianciolo owns shares of Wal-Mart but no other company in this article.