The fund currently trades at an approximate 20% discount to the underlying net asset value of its portfolio.

FORBES: Presidential Cycle Suggests Big Lift Is Coming In Stocks

Since, in most states, the limited partners can't control the portfolio's management or force the partnership's dissolution, an unrelated buyer would demand a discount from the underlying value.

FORBES: Cut your estate tax in half

This is a measure of how much the price of an option will move for a 1% move in the value of the underlying asset.

FORBES: Rogue Trading...Look Past Delta, What About Vega?

Plus there are times when the prices of ETFs veer away from the value of the underlying holdings, which can eat into or add to an investor's return.

WSJ: ETFs for Investors Who Buy and Hold