Ruodu Wang

Bio: Ruodu Wang is an academic researcher from University of Waterloo. The author has contributed to research in topics: Expected shortfall & Coherent risk measure. The author has an hindex of 25, co-authored 154 publications receiving 2632 citations. Previous affiliations of Ruodu Wang include Georgia Institute of Technology & Carnegie Mellon University.

An Academic Response to Basel 3.5

Abstract: Recent crises in the financial industry have shown weaknesses in the modeling of Risk-Weighted Assets (RWAs). Relatively minor model changes may lead to substantial changes in the RWA numbers. Similar problems are encountered in the Value-at-Risk (VaR)-aggregation of risks. In this article, we highlight some of the underlying issues, both methodologically, as well as through examples. In particular, we frame this discussion in the context of two recent regulatory documents we refer to as Basel 3.5.

Aggregation-robustness and model uncertainty of regulatory risk measures

Abstract: Research related to aggregation, robustness and model uncertainty of regulatory risk measures, for instance, value-at-risk (VaR) and expected shortfall (ES), is of fundamental importance within quantitative risk management. In risk aggregation, marginal risks and their dependence structure are often modelled separately, leading to uncertainty arising at the level of a joint model. In this paper, we introduce a notion of qualitative robustness for risk measures, concerning the sensitivity of a risk measure to the uncertainty of dependence in risk aggregation. It turns out that coherent risk measures, such as ES, are more robust than VaR according to the new notion of robustness. We also give approximations and inequalities for aggregation and diversification of VaR under dependence uncertainty, and derive an asymptotic equivalence for worst-case VaR and ES under general conditions. We obtain that for a portfolio of a large number of risks, VaR generally has a larger uncertainty spread compared to ES. The results warn that unjustified diversification arguments for VaR used in risk management need to be taken with much care, and they potentially support the use of ES in risk aggregation. This in particular reflects on the discussions in the recent consultative documents by the Basel Committee on Banking Supervision.

Risk aggregation with dependence uncertainty

Abstract: Risk aggregation with dependence uncertainty refers to the sum of individual risks with known marginal distributions and unspecified dependence structure. We introduce the admissible risk class to study risk aggregation with dependence uncertainty. The admissible risk class has some nice properties such as robustness, convexity, permutation invariance and affine invariance. We then derive a new convex ordering lower bound over this class and give a sufficient condition for this lower bound to be sharp in the case of identical marginal distributions. The results are used to identify extreme scenarios and calculate bounds on Value-at-Risk as well as on convex and coherent risk measures and other quantities of interest in finance and insurance. Numerical illustrations are provided for different settings and commonly-used distributions of risks.

Bounds for the sum of dependent risks and worst Value-at-Risk with monotone marginal densities

Abstract: In quantitative risk management, it is important and challenging to find sharp bounds for the distribution of the sum of dependent risks with given marginal distributions, but an unspecified dependence structure. These bounds are directly related to the problem of obtaining the worst Value-at-Risk of the total risk. Using the idea of complete mixability, we provide a new lower bound for any given marginal distributions and give a necessary and sufficient condition for the sharpness of this new bound. For the sum of dependent risks with an identical distribution, which has either a monotone density or a tail-monotone density, the explicit values of the worst Value-at-Risk and bounds on the distribution of the total risk are obtained. Some examples are given to illustrate the new results.

Convergence of Probability Measures

Abstract: Convergence of Probability Measures. By P. Billingsley. Chichester, Sussex, Wiley, 1968. xii, 253 p. 9 1/4“. 117s.

Lévy processes and infinitely divisible distributions

Abstract: Preface to the revised edition Remarks on notation 1. Basic examples 2. Characterization and existence 3. Stable processes and their extensions 4. The Levy-Ito decomposition of sample functions 5. Distributional properties of Levy processes 6. Subordination and density transformation 7. Recurrence and transience 8. Potential theory for Levy processes 9. Wiener-Hopf factorizations 10. More distributional properties Supplement Solutions to exercises References and author index Subject index.

Capital Regulation, Risk-Taking and Monetary Policy: A Missing Link in the Transmission Mechanism?

Abstract: Few areas of monetary economics have been studied as extensively as the transmission mechanism. The literature on this topic has evolved substantially over the years, following the waxing and waning of conceptual frameworks and the changing characteristics of the financial system. In this paper, taking as a starting point a brief overview of the extant work on the interaction between capital regulation, the business cycle and the transmission mechanism, we offer some broader reflections on the characteristics of the transmission mechanism in light of the evolution of the financial system. We argue that insufficient attention has so far been paid to the link between monetary policy and the perception and pricing of risk by economic agents - what might be termed the "risk-taking channel" of monetary policy. We develop the concept, compare it with current views of the transmission mechanism, explore its mutually reinforcing link with "liquidity" and analyse its interaction with monetary policy reaction functions. We argue that changes in the financial system and prudential regulation may have increased the importance of the risk-taking channel and that prevailing macroeconomic paradigms and associated models are not well suited to capturing it, thereby also reducing their effectiveness as guides to monetary policy.

The fundamental principles of financial regulation

Abstract: Today's financial regulatory systems assume that regulations which make individual banks safe also make the financial system safe. The eleventh Geneva Report on the World Economy shows that this thinking is flawed. Actions that banks take to make themselves safer can - in times of crisis - undermine the system's stability. The Report argues for a different approach. What is needed is micro-prudential (i.e. bank-level) regulation, macro-prudential (i.e. system-wide) regulation, and careful coordination of the two. Macro-prudential regulation in particular needs reform to ensure it countervails the natural decline in measured risk during booms and its rise in subsequent collapses. "Counter-cyclical capital charges" are the way forward; regulators should adjust capital adequacy requirements over the cycle by two multiples - the first related to above-average growth of credit expansion and leverage, the second related to the mismatch in the maturity of assets and liabilities. Changes to mark-to-market procedures are also needed. Macro- and micro-prudential regulation should be carried out by separate institutions since they differ in focus and expertise required. Central Banks should be tasked with macro-prudential regulation, Financial Services Authorities with micro-prudential regulation. Improved international coordination is also important. Since financial and asset-price cycles differ from country to country, counter-cyclical regulatory policy needs to be implemented mainly by the "host" rather than the "home" country.