Shares of Costco (NASDAQ:COST) are down today after the company reported a 15% drop in profit. Unusually for the company, it cited weak gross margins as the source of the profit slump, something many investors didn't expect from Costco and its membership-based business, which usually has more robust gross margins than the traditional retailers.

In this video from Thursday's Stock of the Day, Motley Fool analyst Simon Erickson says he sees the gross margin weakness as coming from overly aggressive discounting to move inventory, which directly affected the company's bottom line. Same-store sales and revenue were both up this quarter, however, which raises the question: What is the most important metric to focus on when understanding Costco?

Simon says that for Costco the most important metric continues to be membership growth. The company added another 4.3 million members in total last year, which he still sees as a healthy number that should inspire investor confidence. While the stock looks more expensive today than a traditional big-box retailer from a price-to-earnings perspective, Simon still likes its membership model better than traditional retail-business models, and still sees the stock as a long-term buy today.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.