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Number of questions: 9
Equipment needed: Useful distribution densities at the end of this question paper
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2 MA318-6-AU 1. The table below shows the increment claims paid on a portfolio of insurance policies. Claims are fully paid by the end of development year 3. Policy year / Development year 0 1 2 3 2008 (year 1) 100 35 30 10 2009 (year 2) 89 20 16 2010 (year 3) 112 33 2011 (year 4) 132 We assume that the above random values are independent and from Poisson(αiβj), with αi, βj > 0, where i is the policy year index and j is the development year index. Use the chain ladder algorithm to estimate the values αi and βj . [10 marks] 2. Claim amounts on a certain type of insurance policy follow an exponential distribution with mean=5, i.e. it has probability density f(x) = 0.2e−0.2x, x > 0. The insurance company purchases a special type of reinsurance policy so that for a given claim X incurred by insurer, the reinsurance company pay 0 if 0 < X < 3 0.5X − 1.5 if 3 ≤ X < 10 X − 6.5 if X ≥ 10 to the insurance company. Calculate the expected amount paid by the reinsurance company on a random chosen claim incurred by insurer. [12 marks] 3. A car insurance company has a particular policy. Denote N as the number of claims per annum and we assume that N follows a Poisson distribution with parameter µ. Claim amounts (X1, · · · , XN ) are assumed to be independent from each other, with exponential distribution λe−λx. Let S = ∑N i=1Xi denote the aggregate annual claims from this policy. A check is made for ruin only at the end of the year. Suppose that the premium income per year is c. (a) Derive the moment generating function of N , Xi and S. [9 marks] (b) Suppose that we can obtain the estimates for the mean and variance of S and denote them as m1 and m2, respectively. If we use a normal approximation to the distribution of S, briefly explain how to calculate the initial capital, U , required in order that the probability of ruin at the end of the first year is 0.01. [3 marks] 3 MA318-6-AU 4. (a) Determine the Jeffreys non-informative prior for the parameter β of a distribution function with density f(x|α, β) ∝ βαxα−1e−βx where α is supposed to be given. [8 marks] (b) Explain in what sense Jeffreys prior is non-informative. [2 marks] 5. Assume that we have an independent sample X1, X2, · · · , Xn from a geometric distribution with parameter p, where the mass function is: f(x|p) = p(1− p)x, 0 < p ≤ 1, x = 0, 1, 2, ... The prior density function for p is given by f(p) = Γ(8) Γ(5)Γ(3) p4(1− p)2, 0 < p < 1 a. Find the posterior distribution for p. [4 marks] b. Show that, under the prior distribution, E ( 1−p p ) = 3 4 . [8 marks] 6. Suppose the weather of a Sunday can be one of three possibilities, θ1 (rainy), θ2 (windy) or θ3 (sunny) and a person can choose one of the three actions to do on that day a1 (picnic), a2 (shopping) or a3 (waching football match). Through consideration, he arrives at the following loss matrix a1 a2 a3 θ1 4 1 2 θ2 0 1 5 θ3 1 2 0 Suppose an action δ of the person must have the form δ = p1 < a1 > +p2 < a2 > +p3 < a3 >, which means with probability pk the person takes action ak, k = 1, 2, 3. Find the best δ based on the minimax rule. [15 marks] 7. Suppose that we observe one data point x ∼ N(θ, σ2) where σ2 is known. The prior distribution is θ ∼ N(0, 1). Find the (prior) predictive distribution. [12 marks] 4 MA318-6-AU 8. The waiting time for a bus at a given station at a certain time of a day is known to have a uniform, U(0, θ), distribution. It is desired to test H0 : 0 < θ < 12 versus H1 : θ > 12. From other similar routes, it is known that θ has a Pareto distribution P(5, 3). If the waiting times 10, 3, 2, 5, 8 are observed, calculate the posterior probability of each hypothesis and the Bayes factor. (You are given the following information: The Pareto P(x0, α) distribution has a pdf, f(θ|x0, α) = α x0 (x0 θ )α+1 I(x0,∞)(θ) with θ > x0, 0 < x0 <∞ and α > 0. It has mean αx0/(α− 1) if α > 1.) [12 marks] 9. Suppose we obtain a posterior which is a Beta(5,8) distribution. The following R code estimates the mean of this posterior distribution. Explain each step of the following R code and explain why it provides a consistent estimate for the mean of this posterior. n = 1000 alpha = 5 beta =8 x = rbeta(n, alpha, beta) mean(x) [5 marks] END OF PAPER MA318 – useful distribution densities 1 p follows a Beta-distribution, Beta(; ), then it has probability density function (pdf) f(pj; ) = 1 B(; ) p1(1 p)1; p 2 [0; 1]; ; > 0 with mean =( + ). Here B(; ) = ()() (+ ) and () has the following property ( + 1) = (): 2 p = (p1; ; pk) follows a Dirichlet-distribution, Dirichlet(1; ; k), then it has pdf f(p1; ; pkj1; ; k) = ( P i i) (1) (k)p 11 1 pk1k ; X pi = 1; pi 2 [0; 1]; i > 0 with mean k= ( P i i) for pk. 3 X follows a Binomial-distribution, then it has pdf f(xjn; p) =
n x
px(1 p)nx; p 2 [0; 1] with mean np. 4 follows a Normal distribution of mean and variance 2, then it has pdf f(j; 2) = 1p 22 e ()2 22 2 MA318 – useful distribution densities 5 X follows a Negative binomial (NB) distribution, NB(r; p), then it has pdf f(xjr; p) =
x+ r 1 x
(1 p)rpx Note that the NB random variable X means, in a sequence of i.i.d. Bernoulli trials, the number of successes before r failures occur. 6 X = (X1; ; Xk) follows a multinomial distribution,M(n;p), then it has pdf f(xjn;p) = n! x1! xk!p x1 1 pxkk where P i xi = n, pi 2 [0; 1] and P i pi = 1. 7 follows a Gamma distribution, Gamma(; ), then it has pdf f(j; ) =
() 1e; ; > 0 Note that if = 1, the gamma distribution becomes an exponential distribution. 8 X follows a uniform distribution, Uniform(a; b) (or using notation U [a; b], uniform[a; b]), then it has pdf f(xja; b) = 1 b a; for all x 2 [a; b]; otherwise f(xja; b) = 0. 9 X follows a Poisson(), then it has probability mass function p(xj) = x x! e;