In the latest string of write-downs caused by the mortgage mess, the Wall Street Journal reports that UBS (NYSE: UBS) may take a $7.11 billion write-down in the fourth quarter and that analysts expect that Citigroup (NYSE: C) has not yet finished announcing its write-downs. The big problem all the banks and brokerage houses are facing is that no one really knows how to value these CDOs because not all CDOs are the same. The underlying assets may or may not be at risk of default.

Merrill Lynch (NYSE: MER) took a more conservative view and wrote down its losses on these assets more significantly than Citigroup because it put lower values on some of its CDOs. Analysts expect that Citigroup, UBS and others may have to follow Merrill Lynch's lead by the end of the year as the underlying values of the mortgage securities they are holding comes to light.

In reality, no one can be sure of the value a CDO until they can actually sell it. There are no set rules on how to value these instruments. Until the mortgage mess started most of these holdings were considered safe investments and rated AAA. Obviously, the ratings agencies need to get their act together and come up with a standard. But the only thing any of us can know for certain is that the values will continue to drop until the mortgage crisis eases and we see a slow down in foreclosures.

Lita Epstein has written more than 20 books including the "Reading Financial Reports for Dummies."