ST2133 Advanced statistics: distribution theory
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ST2133 Advanced statistics: distribution theory
Section A
Answer all three parts of question 1 (40 marks in total)
1. (a) The probability density function for a random variable X is given by
''' a, 0 ≤ x < 1 ;
'
fx (x) =〈 b, 1 ≤ x < 2 ;
'
'
''( 0, otherwise.
i. You are given that E(X) = 5/6. Find a and b. [6 marks]
ii. Find the cumulative distribution function (CDF) of X . [5 marks]
iii. Find P (|X − 1| > 0.5). [4 marks]
(b) Let W = ∑ Xi , where the Xi’s are independent and identically distributed Bernoulli random variables with probability of success p. The random variable N has a Binomial distribution with parameter n and probability of success p, and is independent of the Xi’s.
i. Work out the moment generating function of W . You can use standard formulae for random sums, as long as you can state them clearly.
. [9 marks]
ii. State the probability mass function of W . [3 marks]
(c) At a bus stop, there are two buses available (bus numbers 1 and 2). For i = 1, 2, the waiting time until bus i arrives is denoted by Ti , with T1 independent of T2 , and Ti has density function
fTi(t) = λi exp( −λi t), t > 0.
You are waiting for either bus 1 or 2 to arrive. Let W denote the waiting time until you can get on a bus.
i. Explain briefly why W = min(T1 , T2 ). [2 marks]
ii. Show that E(W) ≤ 1/ max(λ1 , λ2 ). (Hint: You need to work out E(T1 ) and E(T2 ) first.) [5 marks]
iii. Work out the probability density function of W . (Hint: Find P (W > w) first for w > 0.) [6 marks]
Section B
Answer all three questions in this section (60 marks in total)
2. Let X be a Chi-squared random variable with density function
fx (x) = exp ( − ), x > 0.
Suppose Y is independent of, and has the same distribution as, X .
(a) Let Z = ^X . Show that the density function of Z is given by
fz (z) = 2a exp ( − ) , z > 0.
Hence state the value of a. [5 marks]
(b) Let V = X + Y and W = X/Y . Show that the joint density of V and W is
given by
(c) Show that V
fv,w (v, w) = e−u/2, v, w > 0. [9 marks]
and W are independent. State the distribution of V . [6 marks]
3. Consider throws of a biased coin with probability p of showing a head and q = 1 −p
of showing a tail. Assume that throws are independent of each other, and let X denote the number of throws until you obtain a head.
(a) Write down the probability mass function of X . [3 marks]
(b) Find the probability of observing an even number of tails before the first head (0 is counted as even). [4 marks]
(c) Now consider the following game. In each round you throw the biased coin until you get a head, which is the end of the round. If the number of tails before the head is odd, the game ends. If it is even, you start another round. Let N denote the total number of rounds you play.
i. Show that the probability mass function of N is given by
pN (n) = Qn − 1 P, n = 1, 2, . . . ,
where Q and P have to be specified in terms of p and q . [5 marks]
ii. Let Yi denote the total number of throws required to obtain the ith head
(after obtaining the (i − 1)th head), i = 1, . . . , N .
Let W be the total number of throws when the game ends. Write W in
terms of the Yi’s. [2 marks]
iii. If E(Yi ) = µ for i = 1, . . . , N − 1 and E(YN ) = γ, write E(W) in terms of
µ, γ , p |
and |
q . |
[6 marks] |
4. Suppose there are N multiple-choice questions in an examination. Each question has 4 choices. You have a probability of 0.6 of knowing the correct answer to a particular question. If you do not know the answer, you pick one at random. Your answer to different questions are independent of each other.
(a) For a particular question, find the probability that you answer it correctly.
. [3 marks]
(b) Suppose N = 3. Given that you have answered all questions correctly, what is the probability that you only know the answer to exactly two questions?
. [5 marks]
(c) The time T to answer a particular question has the following density function:
fT (t) =〈' λ exp(−λt), '( 4λ exp(−4λt),
if you know the answer;
otherwise.
i. Find E(T). You can use the mean of an exponential distribution without proof, but you need to state it clearly. . [5 marks]
ii. Let Ti be the time to answer the ith question. The Ti’s are independent of each other and identically distributed to T .
Let W be the total time to answer all questions. Find E(W) and Var(W) if you are given that Var(T) = 19/(25λ2 ), E(N) = Var(N) = 20. [7 marks]
2023-05-08