The risk weighted bank capital requirements should at least, as a minimum, have been based on conditional probabilities. They weren’t.

Here a set of tweets on P(A/B)

In probability theory, conditional probability is a measure of the probability of an event (A) occurring (like bankers lending too much to someone safe), given that another event (B) has occurred (that bankers had perceived that someone as very safe).

In probability theory, conditional probability is a measure of the probability of an event (A) occurring (like bankers lending too much to someone risky), given that another event (B) has occurred (that bankers had perceived that someone as very risky).

Any regulators knowing something about conditional probability would never have assigned, for the purpose of risk weighted bank capital requirements, a risk weight of 20% to the very safe AAA rated, and one of 150% to the very risky below BB-rated.

Not the first time I've written about it

PS. Here is my letter to the Financial Stability Board