Also called comparable-store sales, or for those really hip to the lingo, comps. They measure the change from year-to-year of a retailer's sales, in a fixed base of stores, for a given month. Only stores open for at least a year -- some companies extend the time frame to as long as 18 months -- are included in the same-store-sales count. This prevents the number from getting distorted by newer stores that generated disparate sales from a company's core average. It keeps the comparison apples-to-apples.