October 24th, 2002, 12:41 pm
Thank you all for such an interesting thread.ITO 33 is directly concerned with all the topics and issues that you raise, so I can only apologize for having not shared my thoughts with you earlier.I had previously discussed a lot of CB issues on the Wilmott forum, so by all means, try to search for the keyword "Convertible", or "credit spread", or "hazard rate".The good news is that in the next issue of the Wilmott magazine, an article I co-authored will exactly tackle the issues of full or blended discounting in the CB pricing models.All the questions, whether we should apply the credit spread to the bond component (or more generally to any cash-flows, fixed or contingent, expected from the issuer: early call, early put, etc.), or apply it to the difference between the CB price and the conversion option, or apply it to the full CB, and the question whether the share should grow at risky rate, or risk free rate, etc., all these questions are unified, and explained in the light of a general model for credit risky derivatives under default risk. Every particular model (answering yes to any of the previous questions) falls under this general umbrella. Differences are explained in terms of what different RECOVERY assumptions you make.Next, the issue of correlation of credit spread and stock.I agree, unless you want to rely on a structural model and commit to its theoretical assumptions, you have no choice but to specify exogeneously the way credit spread will depend (deterministically or stochastically) on the equity.As a matter of fact, the real function you want to model is not credit spread as a function of equity (for the notion of credit spread is instrument relative: are we talking about the credit spread of a discount bond, of a credit default swap, of an asset swap, etc. ?), but the hazard rate, or the instantaneous probability of default of the issuer, as a function of equity. The hazard rate is relative only to the issuer, therefore you have in theory to specify only one such function for each issuer. Also, it is the hazard rate function that enters explicitly in the "general pricing model under default risk" that I mention in the previous paragraph. Credit spreads are actually outputs of this model, because you will use the same model to price a CDS, a CB, a Asset swap, or a corporate bond.And now you can see that there are two things that you need to specify in my "general model above", (and this is why the difficulty of CB pricing is twofold):- your recovery assumption (this corresponds to one term in the PDE)- you hazard rate function (this corresponds to a term multiplying the previous one).So the problem is how we specify this hazard rate function.This question has been going on for some time on this forum, and there is no ready answer.True the ITO 33 tool is quite general and open to any recovery assumption and any specification of hazard rate function, but then people like B2 will complain that "if you ever have to exogenously specify more than about 3 parameters in a model then you should probably throw it out".I say: Go ahead and pick your favourite structural model if you wish...Actually, it seems that somebody has already done the job for us. CreditGrades for instance publish whole tables of CDS credit spreads (equivalently cumulative probability of default) for different maturity dates, and different equity levels. Go ahead and use this as input, and if you do not wish to use it to trade, use it to manage your risk, or to see how the greeks of the CB would look like, etc.The only problem is that CB is sensitive to credit risk anywhere, anytime, because its payoff depends explicitly on the share level, and you can convert, or be called, anytime.It is OK to use Credit Grades black box to price a CDS or a corporate bond, for a certain equity level, off the credit spread curve that they produce for that equity level. But how do you price the CB?Ho do you get from their table of credit spreads of finite maturity, to "instantaneous credit spreads" (which is what you really need for the CB)?In other words, given their table C(S,T), is there a way of differentiating it with respect to S and T ?The answer is that this "differential" that you need is not a mathematical differential. It is no other than the hazard rate function!!Indeed, the CreditGrades table is really a table of prices: prices of the CDS, or prices of risky zero coupons (roughly equivalent to cumulative probabilies of default).So the question "How we differentiate" is really a full-blown inverse problem in disguise: How do we find the hazard rate function, to go as parameter in our "general pricing model under default risk" such that the CreditGrades price table is matched?We have actually solved this problem. But you can see that all this is very quickly becoming very complex. Remember B2's complaint.Now the last question of risk neutrality.True, everything I said assumes that the hazard rate function is the risk-neutral instantaneous probability of default.But then it is OK to use it to price CBs, if you are inferring it from market prices in the first place (or what you think market prices should be). In other words, my view of the ideal CB pricing model is:- Calibrate the parameters, or the parametric functions, of your pricing equation (volatility, hazard rate) to any market data you have (or will ideally have): CDS credit spreads, vanilla options prices, and (why not?) CDS swaptions. Then have the CB pricing model tell you how to dynamically hedge with the instruments you used in the calibration...
Last edited by
numbersix on October 23rd, 2002, 10:00 pm, edited 1 time in total.