Stat 475 Life Contingencies I Chapter 5: Life Annuities
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Stat 475 Life Contingencies I Chapter 5: Life annuities Life annuities Here we're going to consider the valuation of life annuities. A life annuity is regularly (e.g., continuously, annually, monthly, etc.) spaced series of payments, which are usually based on the survival of the policyholder. These annuities are important for retirement plans, pensions, structured settlements, life insurance, and in many other contexts. Many of the ideas and notation used in the valuation of life annuities borrow from annuities-certain. As with life insurance, the life-contingent nature of the payments means that the present value of benefits is a random variable rather than a fixed number. 2 Review of annuities-certain Recall the actuarial symbols for the present value of n-year annuities-certain: n X 1 − v n Annuity-immediate: a = v k = n i k=1 n−1 X 1 − v n Annuity-due:a ¨ = v k = n d k=0 Z n n −δt 1 − v Continuous annuity:a ¯n = e dt = 0 δ nm n (m) X 1 1 − v mthly annuity-immediate: a = v k=m = n m i (m) k=1 3 Review of annuities-certain | continued Increasing annuity-immediate: n n X a¨n − nv (Ia) = k v k = n i k=1 Decreasing annuity-immediate: n X n − a n (Da) = (n + 1 − k) v k = n i k=1 For the corresponding accumulated values, we replace a by s in each case. For example: n a n (1 + i) = s n 4 Whole life annuity-due Consider an annuity that pays (x) an amount of $1 on an annual basis for as long as (x) is alive, with the first payment occuring immediately. This type of life annuity is known as a whole life annuity-due. We'll denote the random variable representing the PV of this annuity benefit by Y : Y = 1 + v + v 2 + ··· + v Kx =a ¨ (1) Kx +1 The pmf of Y is given by: P Y =a ¨k = k−1jqx for k = 1; 2; 3;::: 5 Whole life annuity-due EPV The EPV of this annuity is denoted bya ¨x and we can use our general strategy to find this EPV: 1 X k a¨x = v k px k=0 We could also take the expectation of the expression given in (1): Important Relation 1 − A a¨ = E [Y ] = x so that 1 = d a¨ + A x d x x This relationship actually holds between the underlying random variables, not just the expected values. 6 Whole life annuity-due EPV, variance, and recursion Finally, we could calculate the EPV using the standard formula for the expectation of a discrete RV: 1 X a¨x = a¨k+1 k jqx k=0 We can find the variance of Y in a similar manner: 2A − (A )2 V [Y ] = x x d2 (Note that we cannot find the second moment of Y by computing the first moment at double the force of interest.) Further, we have the recursiona ¨x = 1 + v px a¨x+1 7 Term annuity-due Now consider an annuity that pays (x) an amount of $1 on an annual basis for up to n years, so long as (x) is alive, with the first payment occuring immediately. This type of life annuity is known as a term annuity-due. For this type of annuity, PV of the benefit is: a¨ if K ≤ n − 1 Kx +1 x Y =a ¨min(K +1; n) = x a¨n if Kx ≥ n In this case, the EPV is denoted bya ¨x:n EPV for Term Annuity-Due n−1 n−1 1 − Ax:n X X a¨ = = v t p = a¨ jq + p a¨ x:n d t x k+1 k x n x n t=0 k=0 8 Example Suppose that a person currently age 65 wants to purchase a 3-year term annuity due with annual payments of $50; 000 each. Given that p65 = 0:95 p66 = 0:91 p67 = 0:87 i = 7% calculate the expected value and variance of the present value of this annuity benefit. [132,146.91; 503,557,608.1] Now suppose that a life insurance company sells this type of annuity to 100 such people, all age 65, all with the same payment amounts. Assuming these people are all independent, calculate the expected value and variance of the total present value of all of the annuity benefits. [13,214,691; 50,355,760,810] 9 Whole life annuity-immediate Now we'll turn to consider life annuities-immediate, starting with a whole life annuity-immediate. This type of annuity pays (x) an amount of $1 on an annual basis for as long as (x) is alive, with the first payment occuring at age x + 1. We'll denote the random variable representing the PV of this annuity benefit by Y ∗: Y ∗ = v + v 2 + ··· + v Kx = a Kx with pmf ∗ P Y = a k = k jqx for k = 0; 1; 2; 3;::: 10 Whole life annuity-immediate (continued) We have the relationship Y ∗ = Y − 1, where Y is the PV of the whole life annuity-due and Y ∗ is the corresponding PV of the corresponding whole life annuity-immediate. The only difference between these two annuities is the payment at time 0. From this relationship, we can obtain expressions for the EPV and variance of Y ∗ 2A − (A )2 a =a ¨ − 1 V [Y ∗] = V [Y ] = x x x x d2 We also have the recursion relation: ax = v px + v px ax+1 11 Term annuity-immediate Now consider an annuity that pays (x) an amount of $1 on an annual basis for up to n years, so long as (x) is alive, with the first payment occuring at age x + 1. This type of life annuity is known as a term annuity-immediate. For this type of annuity, PV of the benefit is: a if K ≤ n ∗ Kx x Y = a min(K ; n) = x a n if Kx > n In this case, the EPV is denoted by ax:n EPV for Term Annuity-Due n X t ax:n = v t px =a ¨x:n + nEx − 1 t=1 12 Example Assume that E = 0:35; q = 0:3; and A1 = 0:2: 20 x 20 x x:20 Find: 1 Ax:20 [0.55] 2 a¨x:20 [13.21] 3 ax:20 [12.56] 13 SOA Practice Problem #25 Your company currently offers a whole life annuity product that pays the annuitant 12,000 at the beginning of each year. A member of your product development team suggests enhancing the product by adding a death benefit that will be paid at the end of the year of death. Using a discount rate, d, of 8%, calculate the death benefit that minimizes the variance of the present value random variable of the new product. [150,000] 14 Whole life continuous annuity Next we'll consider continuous life annuities, starting with a whole life continuous annuity. This type of annuity pays (x) continuously at a rate of $1 per year for as long as (x) is alive, starting now. Letting Y denote the random variable representing the PV of this annuity benefit, we have: Y = a Tx The EPV for this annuity is denoted ax ... EPV for Whole Life Continuous Annuity Z 1 −δt 1 − Ax ax = e t px dt = 0 δ 2 2 Ax − Ax . and the variance is V [Y ] = δ2 15 Term continuous annuity We can also have a continuous annuity that pays for a maximum of n years, so long as (x) is alive. This type of life annuity is known as an n-year term continuous annuity. For this type of annuity, PV of the benefit is: a if T ≤ n Tx x Y = a min(T ; n) = x a n if Tx > n In this case, the EPV is denoted bya ¯x:n EPV for n-year Term Continuous Annuity Z n ¯ −δt 1 − Ax:n a¯x:n = e t px dt = 0 δ 16 Example Suppose that the survival model for (x) is described by the exponential model with µx = µ 8 x. Let Y be the PV of a continuous whole life annuity payable at a rate of 1 per year to (x). d h 1 i 1 Find a − dµ x (δ+µ)2 2 Assuming that δ = 0:08 and µ = 0:04, find the expected value and variance of Y . [8.333; 13.889] 3 Find the probability that Y < 5: [0.2254] 17 SOA Practice Problem #67 For a continuous whole life annuity of 1 on (x): Tx is the future lifetime random variable for (x). The forces of interest and mortality are constant and equal. a¯x = 12:50 Calculate the standard deviation ofa ¯ . [7.217] Tx 18 SOA Practice Problem #79 For a group of individuals all age x, you are given: 30% are smokers and 70% are nonsmokers. The constant force of mortality for smokers is 0.06 at all ages. The constant force of mortality for nonsmokers is 0.03 at all ages. δ = 0:08 Calculate Var a¯ for an individual chosen at random from this Tx group. [14.1] 19 mthly life annuities We can also analyze life annuities that pay on an mthly basis. First consider an annuity that pays an amount of 1 per year, paid 1 th in installments of amount m at the beginning of each m of a year, so long as (x) lives. K (m)+ 1 1 − v x m The PV of this annuity is Y =a ¨(m) = (m) 1 (m) Kx + m d The EPV of Y is given by (m) 1 (m) 1 − Ax X 1 k E[Y ] =a ¨x = = v m k px d(m) m m k=0 1 We can also consider the \immediate" version: a(m) =a ¨(m) − x x m 20 mthly term annuities Now consider an annuity that pays an amount of 1 per year, paid 1 th in installments of amount m at the beginning of each m of a year, so long as (x) lives, but for a maximum of n years.