風險管理的一項重要工作是pricing,許多人都認為pricing涉及許多數學模型,BS model、CIS一大堆公式,所謂火箭專家是指這些。問題是公式歸公式,如何取得市場數據是一大難題。如果市場夠深度,有足夠的買賣,如股票市場的藍籌股,你我都識pricing:成交價便是市場價,不須複雜的模型。然而,像外匯、股票等成熟市場沒人會覺得有錢賺,太透明了,許多銀行家都這樣說。水至清則無魚,不做這些產品,轉到虛無飄渺的東西。什麼Volatility、Credit Derivative 便出籠了。
所謂volatility、default probability,主要是指概率的東西,從來不易量度。請問0.1%的倒閉機率、跟0.2%的倒閉機率有什麼不同?是主觀判斷還真的是計出來的?有人說,是計的。市場上債券的Credit Spread即可反映市場認為該公司的default probability。問題回到這裡:如果市場有足夠深度,參與者夠多,像外匯市場情況,這說法的可信性較高。然而,真正的情況是,Credit spread後面沒足夠的成交支持,據此估計的default probability,會比我主觀判斷的準嗎?
當次按危機發生後,債券市場沒成交,所有credit spread、各種CDS level,都是斷估,強如Markit的各類 Index,也是靠估,利用此等所謂市場數據估出來的所謂mark to market價,比靠估還不如。今日讀Bloomberg news,見了一句話:
"ABX, CMBX, any kind of X you like, are totally uncorrelated to any kind of underlying market," Swiss Re's Aigrain said at the Dubai conference.一家穌黎世的再保險公司的CEO也說:
"The indices are jsut trading on their own account with no relationship whatsoever to an underlying cash market that's ceased to exist."Index 本身有買賣,往往和真正的市場分開。市場旺時還會有arbitrage,使兩者一致;在現在市況,可能真的會兩不相干,買賣兩閒的所謂市場價(只是level,不像超市般ready to sell),你相信它真的是你的問題。
更恐怖的是,文章中說:
Markit doesn't have data on the amount of trading based on exchanges because the transactions happen outside of exchange, or over-the-counter, the company said in an e-mailed statement.Markit group 的 Head of data products and analytics Kevin Gould在談及有關利用ABX或CMBX指數來估值時說:
"We don't think it's particularly relevant to use the index to value assets. It's a very useful tool for understanding the direction the market is moving in, but it is not necessarily a tool that one should use to value the cash asset itself."看了以上的說法,作為風險經理,能盲目地相信指數嗎?以百億美元計的資產、銀行的減值都根據這些指數計算,現在說這些指數不可靠,世間還有更荒謬的事嗎?
這就是全球金融市場。