analysis of the properties of both types of ratings see, for example, Kealhofer et al.
(1998), Carey and Treacy (200), Carey and Hrycay (2001), Kealhofer (2003) and Kou and
Varotto (2008). In this work, we shall explore the different impact of PIT and TTC
ratings on the incremental risk charge.
The IRC should be estimated with a VaR model with a 99.9 per cent confidence level,
which is in line with the VaR parameterisation employed in the banking book (i.e. the
loan book) under the internal rating based approach (IRBA). This way the IRC should
achieve the objective of harmonising the treatment of credit risk in both the trading
book and banking book, and thus reduce the scope of banks choosing strategically to
allocate assets to one book or the other to lower their regulatory capital.
During the recent crisis, securities prices have changed considerably across markets
and types of financial instrument. Although credit risk (in the form or default and
migration risk) has undoubtedly played a role[4], a lot of the price variation was likely
related to market risk factors, such as changes in risk premia (Berg, 2010). This
conclusion is also consistent with the finding in Elton et al. (2001) who shows how risk
premia may have a larger effect on bond returns than default risk factors. To address
this point, the Basel Committee has introduced, on top of the IRC, another capital
add-on designed to make bank capital able to absorb sharp negative price changes in a
crisis. Price risk is measured with a value-at-risk model estimated under stressed
market conditions.
To arrive at the total capital for the trading book, banks will need to add the IRC and
stressed VaR to the current VaR of their trading portfolios. If the internal VaR model
used by the bank does not capture firm specific risks, then those will also need to be
added separately (BCBS, 2009c, p. 18, paragraph 718 LXXXVII-1-). In summary, the old
and new capital requirements will be given by:
Old capital requirement ¼ Current VaR þ Specific risk charge.
New capital requirement ¼ Current VaR þ Specific risk charge þ IRC þ Stressed
VaR.
A separate add-on for specific risk will be necessary when banks compute the VaR
with factor models that only capture the systematic risk component of returns. The
specific risk component may not be modelled on the assumption that it is diversified
away. In our analysis we shall not employ factor models but compute VaR directly on
the empirical return distribution of the portfolios we consider. As a result, any residual
specific risk will be already accounted for in the “Current VaR”.
2.2 The IRC
We estimate the IRC with the IRBA model in Basel II. This is consistent with the Basel
Committee requirement that “[t]he bank must demonstrate that the approach used to
capture incremental risks [i.e. the IRC] meets a soundness standard comparable to that
of the IRBA approach for credit risk...” (BCBS, 2009c, statement 718, XCIII, p. 20). The
IRBA capital requirement K
z;g
for a corporate exposure z with rating g is:
K
z;g
¼ CF · MA
z;g
·12 a
z
ðÞP
D;1;g
2 1 2 a
z
ðÞP
A;1;g
· EAD
z
ð1Þ
where CF is a calibration factor, currently equal to 1.06, which was introduced by the
Basel Committee to make the IRBA deliver, on average across the banking sector, a
Liquidity risk
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