Here are two tiny little puzzles about Moody’s's’s downgrade of the European Financial Stability Facility from Aaa to Aa1 just now. But first, here is some math on EFSF guarantees; basically every €100 of EFSF bonds has €165 of member guarantees, of which €103ish were Aaa-rated and €62ish were not. Until Moody’s downgraded France last week. Now it appears that each €100 EFSF bond has only €67 of Aaa guarantees, €36 of Aa1, and €62 of … various lesser things.
So the puzzles: first, this thing – the EFSF – is basically a structured credit product that is roughly two-thirds guaranteed by a Aaa thing, one-third guaranteed by an Aa1 thing, and roughly another two-thirds guaranteed by an assorted lower-rated miscellany that you can safely ignore. Should that make it (1) Aaa, (2) Aa1, or (4) other? S&P, as it happens, has a mechanism to sort of solve this, which is to say that a bond is rated by its probability of defaulting. Discarding the cats and dogs (and ignoring correlation questions), something that is 1/3 AA+ and 2/3 AAA has about an AA+ chance of defaulting: even if those AAAs are rock-solid, a default by that AA+ counts 100% as a default. Moody’s doesn’t have that – they, in theory, rate structured products1 based on expected loss, not just chances that there will be a default. So something that is two-thirds Aaa and one-third Aa1 is … at least arithmetically closer to Aaa than Aa1, is it not? (Especially if you assume the cats and dogs add a little bit of recovery.) But here you are stuck in a granular world: a thing that is two-thirds Germany and one-third France may be better than France, but I guess it’s also worse than Germany, so you gotta pick one or the other and I suppose pessimism is always a good look.