MATH5905 PRACTICE MIDTERM TEST - 2023 - Week 6
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DEPARTMENT OF STATISTICS
PRACTICE MIDTERM TEST - 2023 - Week 6
MATH5905
Time allowed: 135 minutes
1. Let X = (X1 ,X2 , . . . ,Xn ) be i.i.d. Poisson(θ) random variables with density function
e −θ θx
f(x,θ) = x! , x = 0, 1, 2, . . . , and θ > 0.
a) The statistic T(X) = 对 Xi is complete and sufficient for θ . Provide justifi- cation for why this statement is true.
b) Derive the UMVUE of h(θ) = e −kθ where k = 1, 2, . . . ,n is a known integer. You must justify each step in your answer. Hint: Use the interpretation that P(X1 = 0) = e −θ and therefore P(X1 = 0, . . . ,Xk = 0) = P(X1 = 0)k = e −kθ .
c) Calculate the Cramer-Rao lower bound for the minimal variance of an unbiased estimator of h(θ) = e −kθ .
d) Show that there does not exist an integer k for which the variance of the UMVUE of h(θ) attains this bound.
e) Determine the MLE of h(θ).
f) Suppose that n = 5, T = 10 and k = 1 compute the numerical values of the UMVUE in part (b) and the MLE in part (e). Comment on these values.
g) Consider testing H0 : θ ≤ 2 versus H1 : θ > 2 with a 0-1 loss in Bayesian setting with the prior τ(θ) = 4θ2 e −2θ . What is your decision when n = 5 and T = 10. You may use:
\0 2 x12 e −7xdx = 0.00317
Note: The continuous random variable X has a gamma density f with param- eters α > 0 and β > 0 if
f(x;α,β) = xα − 1 e −x/β
and
Γ(α + 1) = αΓ(α) = α!
2. Let X1 ,X2 , . . . ,Xn be independent random variables, with a density f(x;θ) =
where θ ∈ R1 is an unknown parameter. Let T = min{X1 , . . . ,Xn } = X(1) be the minimal of the n observations.
a) Show that T is a sufficient statistic for the parameter θ .
b) Show that the density of T is
fT (t) =
Hint: You may find the CDF first by using
P(X(1) < x) = 1 − P(X1 > x ∩ X2 > x · · · ∩ Xn > x).
c) Find the maximum likelihood estimator of θ and provide justification.
d) Show that the MLE is a biased estimator. Hint: You might want to consider using a substitution and then utilize the density of an exponential distribution when computing the integral.
e) Show that T = X(1) is complete for θ .
f) Hence determine the UMVUE of θ .
2023-04-06