This excerpt taken from the C 10-K filed Feb 23, 2007.
Quoted securities listed on a national securities exchange and that are freely transferable or traded over-the-counter (including derivative instruments) are marked to market at the close of the principal market for such securities or, if no such transaction reports are available, at the mean
between the last published dealer bid and ask quotations. All securities or other investments that are not quoted on any recognized exchange or over-the-counter market, or where in managements reasonable opinion the published quote is considered unreliable or inappropriate, are valued at fair value by the Investment Manager using various methods, including: use of reasonable judgment based on average bid and offer prices based on quotations furnished by dealers or market makers; pricing models utilizing market inputs and volatility assumptions; and reference to the prices of underlying assets.
Because of the inherent uncertainty of valuation for the investments described above, the estimate of fair value may differ from the values that would have been used had a ready market existed, and the difference could be material.