Crash of 2008/Tutorials

Risk-management errors

(for definitions of the terms shown in italics on this page see the glossary on the Related Articles subpage [2]]

The Stochastic assumption

The underlying assumption of the portfolio theory uon which the risk assessment were based is that investment risks are stochastic rather than deterministic - that is to say, the assumption that they arise from the existence or random fluctuations, and not as a consequence of human behaviour. . The assessment were thus inapplicable to risks due to policy errors.

Data limitations

The data used to estimate risk probabilities were subject to several limitations:

• They were taken from the period of historically low economic volatility that started in the early 1980s and came to be known as the "great moderation" , and as such were applicable only on the assumption that such low volatility would continue. The fact that volatility did, in fact increase was recognised but not acted upon. (According to Frank Raiter (former Managing Director and Head of Residential Mortgage-backed Securities Rating at Standard and Poor's) his company had assessed default probabilities using a model based upon the analysis of 900,000 mortgages that had been implemented in 1996, and which did not, therefore, capture the changes in performance brought about by the subsequent increase in the numbers of subprime mortgages . Later - improved and updated - models had been developed, but were not implemented because of budgetary constraints.)

• They were contaminated by the fact that rescue action had averted some downside risks and thus embodied the assumption that similar action would take place in the future. (The president of another credit rating agency said the their ratings had been made on the assumption that there would be government financial support for any major company that ran into in difficulties .)

Tail risk

An explanation for risk-management errors that has been put forward by Andrew Haldane (Head of the Bank of England's Systemic Risk Assessment Department) suggests that they arose from investors' and rating agencies' use of linear models based upon the CAPM (Capital Asset Pricing Model) . Such models assume that risks can be represented by the symmetrical bell-shaped normal distribution, and can give inaccurate results if the true distribution has a "fat tail", as a result of which there is a significant additional tail risk. Earlier work by Raghuram Rajan (Director of Research at the International Monetary Fund) suggested that securitised assets may be expected to involve significant tail risks. . Since the events involving such risks are by definition rare, they cannot be expected to be picked up by models based upon a five or six years' run of data.

Credit rating errors

Several reasons have been put forward for the errors made by the credit rating agencies. A 2007 report to the Board of Moody's spoke of a conflict between ratings quality and the defence of market share, and of the danger that ratings of securities might be influenced by pressure from their issuers .