Costco (COST) reported mixed fiscal fourth quarter results after the market close on Thursday. The wholesale retailer's earnings topped estimates, but revenue of $79.70 billion fell short of the $79.96 billion estimate. As Yahoo Finance Senior Reporter Brooke DiPalma explains, there was one other metric that missed expectations — membership fee income. She explains why investors were keeping a close eye on that number in the video above.

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This post was written by Stephanie Mikulich.