Coherent Measures of Risk An Exposition for the Lay Actuary by Glenn Meyers Insurance Services Office, Inc.

Transcription

1 Coherent Measures of Risk An Exposition for the Lay Actuary by Glenn Meyers Insurance Services Office, Inc.

2 1. Introduction I believe the ideas in the paper Coherent Measures of Risk by Philippe Artzner, Freddy Delbaen, Jean-Marc Eber and David Heath [2] merit serious consideration by members of the Casualty Actuarial Society (CAS). However, their paper is written for an academic audience with extensive training in probability theory. The majority of CAS members will have some difficulty digesting the paper itself. This paper is my attempt to describe these ideas in language that is familiar to most CAS members. Their paper is motivated by the problem of setting margin requirements on an organized exchange. This problem is similar to that of setting capital requirements for insurance companies. Artzner [3] has written another paper on the subject that casualty actuaries might find more accessible. 2. A Motivation for the Definition of Coherence Consider the following set of ten scenarios, each with associated losses X 1, X 2, X 3 and X 4. Table 1 Scenario X 1 X 2 X 1 +X 2 X 3 = 2*X 1 X 4 = X Maximum Loss Glenn Meyers Page 1 March 16, 2000

3 We can think of the X i s as random variables representing the losses of the i th risk. In our examples, we shall assume that each scenario is equally likely. Let a measure of risk for X i be defined as r(x i ) = Maximum(X i ), where the maximum is taken over all ten scenarios. This measure of risk fulfills the needs of an insurance regulator who wishes to require that the insurer have sufficient assets to cover the losses incurred in each of the scenarios. Some of the assets may be supplied by premiums paid by the insureds. The remainder of the assets must be supplied as insurer capital. Using Table 1 as an aid, the reader should be able to verify that the measure of risk, r, satisfies the following axioms. 1. Subadditivity For all random losses X and Y, 2. Monotonicity If X Y ( X + Y ) ( X ) + ( Y ) ρ ρ ρ for each scenario, then, ρ ( X) ρ( Y) 3. Positive Homogeneity For all λ 0 and random losses X, ( X) = λρ ( X ) ρ λ 4. Translation Invariance For all random losses X and constants a. ( X ) ( X) ρ + α = ρ + α A risk measure that satisfies these four axioms is called a coherent measure of risk. Glenn Meyers Page 2 March 16, 2000

4 These properties of a risk measure appear to be reasonable. Consider the following remarks. Subadditivity reflects the diversification of portfolios, or that, a merger does not create extra risk [4, 4 th page] and [2, 6 th page]. This is a natural requirement consistent with the role of insurance. Positive homogeneity is a limiting case of subadditivity, representing what happens when there is precisely no diversification effect [4, 4 th page]. Glenn Meyers Page 3 March 16, 2000

5 3. Other Measures of Risk It turns out that many common measures of risk used by actuaries are not coherent. Consider the following examples. Define the Value at Risk or VaR as the smallest loss that is greater than a predetermined percentile of the loss distribution. This measure is similar to Probability of Ruin measures that actuaries have long discussed. If our measure of risk, r(x), is the 85 th percentile of the random loss X, we have for the scenarios listed in Table 2 below: ( X ) ( X ) ( X X ) 0= ρ + ρ < ρ + = As this example shows, the value at risk criterion violates the subadditivity axiom. Table 2 Scenario X 1 X 2 X 1 +X VaR@85% Glenn Meyers Page 4 March 16, 2000

6 The Standard Deviation criterion sets the measure as the expected value of the loss plus a predetermined multiple of the standard deviation. For the scenarios listed in Table 3 below we have: ρ ρ X X 1 2 ( X1) E[ X1] StDev[ X1] ( X ) E[ X ] StDev[ X ] + 2 = = As this example shows, the standard deviation principle violates the monotonicity axiom. Table 3 Scenario X 1 X E[Loss] StDev[Loss] E[Loss]+2*StDev[Loss] So far, I have demonstrated that two popular statistical measures discussed in the literature (if not used in practice) on solvency standards are not coherent. Let me now turn to a more general description of measures of risk that are coherent. Glenn Meyers Page 5 March 16, 2000

7 4. The Representation Theorem Let W denote a finite set of scenarios. Let X be the loss incurred by the insurer under a particular business plan. We associate each loss with an element of W. The representation theorem [2, Proposition 4.1 and 3, Proposition 2.1] states that a risk measure, r, is coherent if and only if there exists a family, P, of probability measures defined on W such that ρ ( X) = sup{ EP [ X] P P }. (1) One way to construct a family of probability measures on W is to take a collection m { A i} i = 1 A = of subsets of W with the property that m i= 1 A i =Ω. Let n i be the number of elements in A i. Assume that all elements in W are equally likely. We then define the probability measure, P i, on the elements w Œ W as the conditional probability given that the element is in the set A i, and 0 otherwise. That is 1 if ω Ai i ( ) n P ω = i. 0 if ω A i The authors [2, 16 th page] refer to the collection of probability measures, P, on the set of scenarios as generalized scenarios. Glenn Meyers Page 6 March 16, 2000

8 Let s look at an example. The following table gives a set of scenarios and associated losses. Table 4 Scenario X Let A 1 = {1,2} and A 2 = {3,4}. We then calculate the expected values P [ ] and P [ ] E X = 1 1 E X = 4. 2 The associated coherent measure of risk, r A (X), is then given by { } ( ) P [ ] ρ X = sup E X i = 1,2 = 4. A i We can similarly construct a second coherent measure of risk, r B (X), on the scenarios in Table 4 with the subsets B i = {i}. In this case we have r B (X)= 6. One can impose varying degrees of conservatism on coherent measures of risk by varying the choice of generalized scenarios. Glenn Meyers Page 7 March 16, 2000

9 5. A Proposal for a Risk Measure Artzner, Delbaen, Eber and Heath s paper finishes with a proposal for a risk measure that casualty actuaries should find easy to implement. Let s start with the formal definition of the Value at Risk (VaR). Let a be a selected probability (e.g. 99%). Then { α } ( ) inf Pr{ } VaR X = x X x > α As demonstrated above, VaR is not a coherent measure of risk. We now define the proposed measure in terms of the VaR. We call this measure the Tail Conditional Expectation (TCE) or Tail Value at Risk (TailVaR). ( ) ( ) ( ) TCEα X TailVaRα X E X X VaRα X The TailVaR is linked to a well-known criterion in recent CAS literature for solvency - the Expected Policyholder Deficit (EPD). See, for example, [1] and [5]. EPD(t) is defined as the expected loss over a predetermined threshold t. We then have that ( α ( X) ) EPD VaR TailVaRα ( X) = VaRα ( X) + 1 α. Glenn Meyers Page 8 March 16, 2000

10 I will now demonstrate that the TailVaR is coherent under some common conditions. For any subset A of W, let n A be the number of elements in A. Define the probability measure Proposition 1 if ω A A ( ω ) n P = A. 0 if ω A If the probability of each element of W is equally likely, then the TailVaR is a coherent measure of risk. Proof Let n be the number of elements in W. Denote the various values of X by x1 x2... xn. Let k be the integer with 0 k < nsuch that k + 1 k+ 1 Since Pr{ X x } = > α and Pr{ X x } n Let A be the family of subsets of W with exactly n measures P { P A }. By Equation 1, ( ) [ ] P measure of risk. = A A For any scenario w Œ A, Pr{ W ωω A} k k+ 1 α, n n. k < k + 1 α we have that VaR a (X) = x k+1. n k elements. Define the family of ρ X = sup{ E X A A } is a coherent 1 = =. n k Let AMax be the member of A with the n k largest elements, i.e.{ x, x,..., x } We then have ( ) = E ( ) TailVaRα X X X VaRα X xk+ 1+ xk xn = n k E. = P AMax For any other set A ŒA, E [ X] E [ X] Thus ( ) sup EP [ ] PA P. AMax { } A [ X] TailVaRα X = X A A and the risk measure is coherent. A k+ 1 k+ 2 n. Glenn Meyers Page 9 March 16, 2000

11 Note This proposition is slightly more general than [2, Proposition 5.3]. I provided the proof above for completeness. It is a minor modification of the proof that is given in [2]. Many actuaries use a continuous distribution function, with perhaps with a small number of jump discontinuities, to model the aggregate losses of an insurer. Since this distribution can be approximated, to any desired degree of closeness, with a distribution based on a number of equally likely scenarios, it is correct to say that the TailVaR measure applied to such a distribution is coherent. Glenn Meyers Page 10 March 16, 2000

12 References 1. American Academy of Actuaries Property/Casualty Risk-Based Capital Task Force, Report on Reserve and Underwriting Risk Factors, Casualty Actuarial Society Forum, Summer 1993 Edition. 2. Philippe Artzner, Freddy Delbaen, Jean-Marc Eber and David Heath, Coherent Measures of Risk, Math. Finance 9 (1999), no. 3, Philippe Artzner, Application of Coherent Risk Measures to Capital Requirements in Insurance, North American Actuarial Journal, Volume 3, Number 2, April Michael Denault, Coherent Allocation of Risk Capital, Risklab web site Glenn Meyers, Underwriting Risk, 1999 Spring Forum, Casualty Actuarial Society Glenn Meyers Page 11 March 16, 2000