AN INFORMATION-BASED APPROACH TO CREDIT-RISK MODELLING. by Matteo L. Bedini Universitè de Bretagne Occidentale

Transcription

1 AN INFORMATION-BASED APPROACH TO CREDIT-RISK MODELLING by Matteo L. Bedini Universitè de Bretagne Occidentale

2 Agenda Credit Risk The Information-based Approach Defaultable Discount Bond Dynamics Derivatives and Coupon Bond Considerations on the Model

3 Agenda Credit Risk The Information-based Approach Defaultable Discount Bond Dynamics Derivatives and Coupon Bond Considerations on the model

4 Credit Risk Definition In financial markets credit risk is the risk associated to the possibility that a counterparty in a financial contract will not fulfill a contractual commitment to meet her/his obligation stated in the contract. EXAMPLES PARMALAT LEHMAN BROTHERS 4/31

5 Credit Risk Mathematical Finance and Credit Risk 1. Problem of modelling: How is Credit Risk described? Structural Models Intensity Models Information-based Models 2. Problem of valuating: Given the model, how is a financial contract valuated? Zero-Coupon Bond Coupon Bond Options Credit Default Swap 5/31

6 Credit Risk Basic Assumptions 1. Default-free interest rate system is deterministic. 2. Financial market is modelled through the specification of a probability space (the probability measure Q is the risk-neutral measure). 3. All processes are adapted to the market filtration. The existence of a unique risk-neutral measure is ensured, 6/31 even if the market may be incomplete.

7 Credit Risk General settings (1/2) Under these hypothesis, if H T represents a cash-flow at time T > 0, then its value H t at time t < T is given by: EXAMPLE: Binary bond. Q(H T =h 1 )=p 1 (no default) 7/31 Q(H T =h 0 )=p 0 =1-p 1 (default)

8 Credit Risk General settings (2/2) A defaultable bond is a financial contract that, at a pre-specified instant of time (maturity), delivers to the owner a certain amount of money, if the default never occurs. The random variable H T represents the final value of the defaultable bond. H T takes value h i with a priori probability p i (i=1,,n): Q(H T =h i )=p i. At time t, the price B tt of a defaultable bond with maturity T>0, is given by: The purpose is to obtain the bond price process: 8/31

9 Agenda Credit Risk The Information-based Approach Defaultable Discount Bond Dynamics Derivatives and Coupon Bond Considerations on the Model

10 The Information-based Approach The information-process (1/2) There exist an F t -adapted process accessible to market agents, modelling the flow of information concerning future cash-flow of the defaultable bond: σ is a constant (information parameter). H T is an F T -measurable random variable. β tt is a standard Brownian bridge on [0, T] independent from H T (it is F T - measurable! ). 10/31 Theorem: ξ t satisfies the Markov property.

11 The Information-based Approach The information-process (2/2) t in (0,T): news, rumors, stories and speculation are mixed together, building the information about H T arriving on the market. 11/31 t=0: all the information is in the a priori probability distributions t=t: the moment of truth.

12 The Information-based Approach Bond Price Process Simplifying assumption: the subalgebra generated by the information process ξ t is the market filtration: 12/31

13 The Information-based Approach Bayes formula 13/31

14 The Information-based Approach Bond price process 14/31 Next step: obtain the defaultable bond dynamics db tt =?

15 Agenda Credit Risk The Information-based Approach Defaultable Discount Bond Dynamics Derivatives and Coupon Bond Considerations on the Model

16 Defaultable Discount Bond Dynamics The Brownian motion Theorem: W t is an F t -Brownian motion. The conditional probability: 16/31

17 Defaultable Discount Bond Dynamics Bond price dynamics: Dynamics The short rate: Absolute bond volatility: Conditional variance: 17/31

18 Defaultable Discount Bond Dynamics Simulations of a digital bond. σ=35% σ=55% 18/31 σ=75% σ=95%

19 Agenda Credit Risk The Information-based Approach Defaultable Discount Bond Dynamics Derivatives and Coupon Bond Considerations on the Model

20 Derivatives and Coupon Bond European call option (1/3) An European call option is a financial contract that gives the owner the right to buy a pre-specified asset (the underlying) at a pre-specified price (the strike price) at a given instant of time. T is the maturity of the defaultable bond. t is the maturity of the option. K is the strike price. 20/31

21 Derivatives and Coupon Bond European call option (2/3) 21/31

22 Derivatives and Coupon Bond European call option (3/3) Change of measure by using factor Φ t : from measure Q to measure B (the bridge measure). Binary case (i=1): 22/31

23 Derivatives and Coupon Bond Numerical results Call option: C 0 = f( B 0 ) Put option: P 0 = f( B 0 ) 23/31 Call option: Δ= C 0 / B 0 Call option: Vega= C 0 / σ

24 Derivatives and Coupon Bond The X-factor Approach Modeling more complex situations: how to describe multiple cash-flow? Idea: if we have n cash-flows, each at time T i, we can built n information processes ξ (i), i=1,,n, describing the information regarding the corresponding cash-flows. 24/31

25 Derivatives and Coupon Bond Credit Default Swap A Credit Default Swap (CDS) is a credit derivative between two counterparties, whereby one makes periodic payments (g) to the other and receives the promise of a payoff (h) if a third party defaults. The former party receives credit protection and is said to be the buyer while the other party provides credit protection and is said to be the seller. The third party is known as the reference entity. It often happen that the coupon g and the payoff h are chosen in such way the value V t of the CDS at time t=0 is V 0 =0. (*) 25/31 (*) In the first formula X tt0 = 1 for convenience

26 Derivatives and Coupon Bond Coupon Bond A Coupon Bond is a contract between a buyer and a seller in which at time t=0 the buyer gives to the seller p euro (principal). The seller will pay to the buyer at some pre-specified dates T 1,, T n a pre-specified amount of money (coupon) c i, i=1,, n, and at time T n the seller will pay even the principal p. 26/30

27 Derivatives and Coupon Bond Numerical simulations Simulation of the dynamics of a 5-years CDS. Earnings are positive for the seller of protection. 27/31 Simulation of the dynamics of a 5-years Coupon Bond.

28 Agenda Credit Risk The Information-based Approach Defaultable Discount Bond Dynamics Derivatives and Coupon Bond Considerations on the Model

29 Consideration on the Model Further development Stochastic default-free interest rate system Final cash-flow (H T ) dependent from the noise Generalized noise process 29/31

30 Consideration on the Model Conclusion A new class of models for Credit-risk has been analyzed. Central role of the information arriving on the market. It is possible to obtain bond price process (relating the a priori probability with the a posteriori). Explicit formula for bond price dynamics. Possibility of pricing derivatives (vanilla options, CDS, ). 30/31

31 Bibliography D. C. Brody, L. P. Hughston & A. Macrina. Beyond Hazard rates: a new framework for credit risk modelling. Advances in Mathematical Finance, Festschrift volume in honour of Dilip Madan. Birkhauser, Basel, T. R. Bielecki and M. Rutkowski. Credit Risk: Modelling, Valuation and Hedging. Springer, P. J. Schonbucher. Credit Derivatives Pricing Models. John Wiley & Sons, 2003 T. R. Bielecki, M. Jeanblanc, and M. Rutkowski. Modelling and valuation of credit risk. In Stochastic Methods in Finance, Bressanone Lectures 2003, eds. M. Frittelli and W. Runggaldier, LNM 1856, Springer D. Lando. Credit Risk Modelling. Princeton University Press, M. Rutkowski and N. Yu. An extension of the Brody-Hughston-Macrina approach to modelling of defaultable bonds. Int. J. Theor. Appl. Fin. 10, , D. C. Brody, M. H. A. Davis, R. L. Friedman, L. P. Hughston, Informed traders. Working paper, D. C. Brody, L. P. Hughston & A. Macrina. Information-based asset pricing. International Journal of Theoretical and Applied Finance. 2008, vol. II, issue 01, pages /31

32 THANK YOU VERY MUCH! Grazie mille!