A linear algebraic method for pricing temporary life annuities

Transcription

1 A linear algebraic method for pricing temporary life annuities P. Date (joint work with R. Mamon, L. Jalen and I.C. Wang) Department of Mathematical Sciences, Brunel University, London

2 Outline Introduction Cash flow valuation problems in actuarial science

3 Outline Introduction Cash flow valuation problems in actuarial science 1. A linear algebraic approach to random cash flow pricing 2. Approximate solutions

4 Outline Introduction Cash flow valuation problems in actuarial science 1. A linear algebraic approach to random cash flow pricing 2. Approximate solutions Applications

5 Introduction: pricing an annuity With no mortality risk, the present value of cash flow f (e.g. purchased temporary life annuity) payable at a future times t i, at time t k,0 k < i < N is p k = N 1 i=k+1 f i i j=k+1 (1 + r j ) r i is one period interest rate in [t i 1,t i ).

6 Introduction: pricing an annuity With no mortality risk, the present value of cash flow f (e.g. purchased temporary life annuity) payable at a future times t i, at time t k,0 k < i < N is p k = N 1 i=k+1 f i i j=k+1 (1 + r j ) r i is one period interest rate in [t i 1,t i ). Example: f = 1, N = 1, p 0 = r 1.

7 Pricing an annuity (cont d) With no interest rate risk but with force of mortality risk, the same present value is p k = N 1 i=k+1 f i i j=k+1 (1 + λ j ). λ i is force of mortality in [t i 1,t i ) λ i = probability that annuitant alive at t i 1 survives till t i.

8 Pricing an annuity (cont d) With both interest rate and mortality risk, p k = N 1 i=k+1 f i i j=k+1 (1 + λ j)(1 + r j ), E(p k ) =? Conventional approaches: 1. Use affine term structure models for both interest rate and force of mortality; 2. More complex models, with simulation-based pricing.

9 Pricing an annuity (cont d) Our approach: 1. Expand p k into a polynomial in λ j, r j ; base an approximation of E(p k ) on the moments of λ j, r j. 2. E(r j ), E(r i r j ) are available (forward curve data - no parametric model needed). Forward mortality information may be available.

10 Pricing an annuity (cont d) Our approach: 1. Expand p k into a polynomial in λ j, r j ; base an approximation of E(p k ) on the moments of λ j, r j. 2. E(r j ), E(r i r j ) are available (forward curve data - no parametric model needed). Forward mortality information may be available. 3. Strong results on convergence of approximation + very good simulation results. 4. Extends to various contracts (mortgage insurance, annuities, f contingent on something else).

11 A linear algebraic approach With discounting factor θ i = r i + λ i + r i λ i, the relationship p 1 = f 2, running present value at t θ 2 p 0 = f 1 f θ 1 (1 + θ 2 )(1 + θ 1 ), = f 1 + p θ 1, may also be written as (1 + θ 2 )p 1 + 0p 0 = f 2, ( 1)p 1 + (1 + θ 1 )p 0 = f 1.

12 A linear algebraic approach (cont d) 1. Thus p = [ p 1 p 0 ] solves a system of linear equations ([ ] [ ]) θ2 0 p = f. 0 θ 1 2. If f are temporary life annuity payments, E(p 0 ) is the lump sum purchase price. 3. More involved formulation possible for insurance contracts; leads to random terms of both sides. 4. For future θ i unknown and random, we have a problem of inverting a structured matrix with random entries.

13 Random matrix inversion problem For a series of cash flows f i,0 i N, we can write f = (Q + Θ)p p is N vector of running present values, f is N vector of cash flows. with θ i = r i + λ i + r i λ i. [Q] ij = 1 if i = j = 1 if i = j + 1 = 0 otherwise [Θ] ij = θ N i+1 if i = j = 0 otherwise, This means the value of the series of cash flows at time t k is [(Q + Θ) 1 f] k.

14 Approximate inversion: scalar case For θ i = θ < 1, f 1 + θ = ( 1 θ + θ 2 ) f M ( θ) i f. i=0 A similar result holds if θ is bounded by unity, but random. Does an analogous expansion hold if θ i θ j,i j?

15 Convergent approximation to the solution: vector case Theorem Suppose that θ i satisfy P( Q 1 Θ < 1) = 1, f i satisfy max i f i < γ, γ < and the inverse of (Q + Θ) exists with probability 1. Define p = (Q + Θ) 1 f, p M = M ( Q 1 Θ) i Q 1 f (1) i=0 with ( Q 1 Θ) 0 = I. Then (i) p M p with probability 1. (ii) ( ) lim E p M p 2 2 = 0. M

16 What does p M look like? It may be shown that [Q 1 Θ] ij = θ N j+1 if i j, = 0 otherwise, j [Q 1 f] j = f N i. i=1 e.g. for N = 3, θ Q 1 Θ = θ 3 θ 2 0 θ 3 θ 2 θ 1 f 3 Q 1 f = f 3 + f 2 f 3 + f 2 + f 1

17 How good is the approximation? With probability 1, p M p Q 1 Θ M+1 1 Q 1 Θ Q 1 f holds for any induced norm, provided Q 1 Θ < 1. If θ i < θ max w.p.1, N(N + 1) Q 1 Θ 2 θ max. 2 Further, given any ǫ > 0, a matrix norm s.t. Q 1 Θ < θ max + ǫ. Non-conservative bounds are difficult; but- the algorithm seems to work well in practice.

18 Putting it all together Finally, Given moments E(r i ), E(λ i ), E(r i r j ), E(λ i λ j ), we can construct a second order approximation for ( N ) f i E i i=1 j=1 (1 + λ, j)(1 + r j ) } {{ } an element from vector E( 2 i=0 ( Q 1 Θ) i (Q 1 f)) in closed-form.

19 Putting it all together Finally, Given moments E(r i ), E(λ i ), E(r i r j ), E(λ i λ j ), we can construct a second order approximation for ( N ) f i E i i=1 j=1 (1 + λ, j)(1 + r j ) } {{ } an element from vector E( 2 i=0 ( Q 1 Θ) i (Q 1 f)) in closed-form. E(r i ) (forward curve), E(r i r j ) (forward curve correlations) may be known, or found from affine term structure model. E(λ i ), E(λ i λ j ) may be found from an affine term structure model for force of mortality.

20 Simulation Experiments Extensive numerical simulation experiments conducted for annuity valuation, insurance premium (Date et al, 2010), general cash flows (Date et al 2007). The method is computationally very cheap and shown to be quite accurate. This requires joint moments of interest rates and mortality up to M th order. These moments may be known (forward curve and historic correlations - no interest rate model needed for M = 2.) or they may be inferred from a given affine term structure model (e.g. for mortality).

21 Main References P. Date, R. Mamon, L. Jalen and I.C. Wang, A linear algebraic method for pricing temporary life annuities and insurance policies, Insurance: Mathematics and Economics, vol. 47, pages , P. Gaillardetz, Valuation of life insurance products under stochastic interest rates, Insurance: Mathematics and Economics, vol. 42, pages , P. Date, R. Mamon and I.C. Wang, Valuation of cash flows under random rates of interest: a linear algebraic approach, Insurance: Mathematics and Economics, vol. 41, pages 84 95, L. Ballotta and S. Haberman, The fair valuation problem of guaranteed annuity options: the stochastic mortality environment case, Insurance: Mathematics and Economics, vol. 38, pages , 2006.