Clarify the relation between DUG and USO.

DUG
The investment seeks daily investment results, before fees and expenses, which correspond to twice the inverse of the daily performance of the Dow Jones U.S. Oil & Gas index. The fund normally invests 80% of assets in financial instruments with economic characteristics that should be inverse to those of the index. It may employ leveraged investment techniques in seeking its investment objective. The fund is nondiversified.

Dow Jones U.S. Oil & Gas index
It is like XLE, but more divresified.
http://finance.yahoo.com/q/cp?s=%5EDJUSEN

USO
The investment seeks to reflect the performance, less expenses, of the spot price of West Texas Intermediate (WTI) light, sweet crude oil. The fund will invest in futures contracts for WTI light, sweet crude oil, other types of crude oil, heating oil, gasoline, natural gas and other petroleum based-fuels that are traded on exchanges. It may also invest in other oil interests such as cash-settled options on oil futures contracts, forward contracts for oil, and OTC transactions that are based on the price of oil. The fund is nondiversified.

Conclusion: XLE may not necessarily track USO, so DUG is not necessarily inverse of USO.