Analysis of mortgages, especially tranches of slices of portions of thousands of mortgages, just isn't that easy. That's the entire reason that companies like A

Analysis of mortgages, especially tranches of slices of portions of thousands of mortgages, just isn't that easy. That's the entire reason that companies like ACA, who Goldman claimed picked the mortgages, exist: to act as an impartial third party that can analyze and rate the risk of the underlying assets when there are too many for the investor to do it herself. If I sold you one diamond, and you were a shrewd buyer, you'd probably take it somewhere, perhaps to two people, to get it examined and ensure its authenticity. But if I offered to sell you a big ol' bag full of thousands of assorted gemstones, and claimed that everything in the bag was either pure diamond, ruby, emerald or opal, 25% each, you probably wouldn't have the time or money to go through each one and analyze its authenticity. Even if you did, my offer may have expired by the time you do. So instead, I say that the gems were all hand-picked and inspected by the American Gem Trade Association, and that they all checked out to be 100% authentic. If you trust me and the AGTA, you might buy the bag at this point. However, when you get home & find out that they're all rhinestones, I guarantee that you will consider my false assertion that they were picked by the AGTA to be both material and criminal.