When it comes to retail stocks, few metrics are more important than same-store sales, which measure how much sales at a retailer's existing locations rise or fall on a monthly, quarterly, or annual basis relative to the year-ago period. Given this, and the fact that historical same-store sales figures are rarely available for free in an aggregated form, I thought it would be useful for investors to have a comprehensive list of Costco's (COST 1.57%) same-store sales over the past eight years.

But before getting to that -- the full table is located at the end of this article -- it's worth noting why same-store sales matter. In short, a retail company can grow in two ways: building new stores or increasing sales at existing locations. Both types of growth are positive, but the latter is better for a business and its shareholders -- and, just to be clear, these are not mutually exclusive.

The preference toward same-store sales growth, sometimes referred to as "organic" growth, at companies like Costco stems from the fact that it's the most efficient way to boost both the top and bottom line. The physical building is already in place, the staff has been hired and trained, and the location has previously been integrated into the company's distribution network. New locations, on the other hand, don't have these advantages. As such, expansion through nonorganic channels costs more.

Same-store sales also help investors determine if a particular retailer is gaining traction with customers. This is important because, at some point, a retailer reaches a saturation point and can't build new stores without severely cannibalizing sales from nearby locations. In that case, the only remaining channel for growth is through increasing revenue at existing locations. One could argue, for instance, that Wal-Mart has reached this point, which explains why it has largely switched to building smaller stores that can patch geographical voids between its Supercenters.

It's for these reasons, in turn, that companies such as Costco rely on same-store sales more than any other figure to boost profitability. As the chain of membership warehouses explained in its latest annual report:

We believe that the most important driver of increasing our profitability is sales growth, particularly comparable sales growth. We define comparable warehouse sales as sales from warehouses open for more than one year, including remodels, relocations and expansions, as well as online sales related to websites operating for more than one year. ... The higher our comparable sales [excluding the impacts from fluctuations in gas prices and foreign currencies], the more we can leverage certain of our selling, general and administrative expenses, reducing them as a percentage of sales and enhancing profitability.

Thus, without further ado, the table below contains Costco's quarterly same-store sales figures going back to the second quarter of 2007:

Quarter

U.S. Locations

International Locations

Total Company

3Q15

1%

-6%

-1%

2Q15

4%

-2%

2%

1Q15

6%

1%

5%

4Q14

6%

6%

6%

3Q14

5%

3%

4%

2Q14

4%

0%

3%

1Q14

3%

1%

3%

4Q13

5%

4%

5%

3Q13

6%

4%

5%

2Q13

5%

6%

5%

1Q13

7%

9%

7%

4Q12

6%

2%

5%

3Q12

5%

5%

5%

2Q12

8%

8%

8%

1Q12

10%

11%

10%

4Q11

10%

19%

12%

3Q11

10%

18%

12%

2Q11

5%

12%

7%

1Q11

5%

14%

7%

4Q10

4%

14%

6%

3Q10

6%

26%

10%

2Q10

5%

26%

9%

1Q10

1%

13%

3%

4Q09

-6%

-3%

-5%

3Q09

-5%

-12%

-7%

2Q09

-1%

-11%

-3%

1Q09

3%

-7%

1%

4Q08

9%

11%

9%

3Q08

6%

16%

8%

2Q08

5%

17%

7%

1Q08

5%

17%

8%

4Q07

4%

9%

5%

3Q07

7%

10%

7%

2Q07

5%

4%

5%

Source: Costco's quarterly earnings releases.