Valero (NYSE:VLO) was the first big refiner to report earnings, doing so earlier this week. In this video, Fool.com contributor Aimee Duffy takes a closer look at the company's gross margin per barrel for its mid-continent operations, which shrank 18% year over year. Aimee discusses how Valero's past numbers have compared to the other big mid-con refiners, and if there is even a point for investors to make the comparison at all.