DEERFIELD – An important Walgreen revenue measurement slid more than analysts expected in December, even though prescription counts continued to recover for the nation’s largest drugstore chain since the resolution of a contract dispute.

The Deerfield company said Friday revenue from stores open at least a year dropped 6.1 percent last month as a shift toward generic drugs and a calendar quirk hurt the company’s sales. Pharmacy revenue tumbled 8.9 percent, while revenue from the front end, or rest of the store fell, 2.3 percent.

Revenue from stores open at least a year is considered a key indicator of retailer health because it leaves out results from locations that have opened or closed in the last year.