With the refining margin and cost structures, you have to take into account that on an annual basis, refineries often have to have downtime for maintenance and have investments made in capital to keep the production facilities at the optimal level to be producing.
There are changes in the variety of crude oils that refineries use that often require them, on a kind of cyclical basis, to change again how they produce gasoline. Refineries are amazingly technologically sophisticated operations that try to extract the most out of every ounce of oil or crude they're putting in. So the cyclical typically has variations related to those aspects.
The other aspect is the source and price at which they're acquiring the crude. I had mentioned earlier the variation between the offshore crude and the crude that's from North American production. We're seeing some variation--what you would see here--in terms of the line that's moving as opposed to the average, which our model hasn't accounted for because of the variation between North American prices and global. It hasn't, for a long period of time, had a huge differential. Right now it's quite substantial and has been increasing over time.
We're looking at adjusting our model. We ran some other aspects to the model before we arrived here just to demonstrate in our minds that the cyclical nature is actually much more smooth when you look at the pricing of crude oil from global and North American markets.
So that's one thing to look at from the perspective of what aspects feed that particular variation.