JAM 2006

MATHEMATICAL STATISTICS TEST PAPER

1
 Special Instructions / Useful Data
1. For an event A, P ( A ) denotes the probability of the event A.
2. The complement of an event is denoted by putting a superscript “c” on the event, e.g. Ac denotes the
complement of the event A.
3. For a random variable X , E ( X ) denotes the expectation of X and V ( X ) denotes its variance.
( )
4. N μ , σ 2 denotes a normal distribution with mean μ and variance σ 2 .
5. Standard normal random variable is a random variable having a normal distribution with mean 0 and
variance 1.
6. P ( Z > 1.96 ) = 0.025, P ( Z > 1.65 ) = 0.050, P ( Z > 0.675 ) = 0.250 and P ( Z > 2.33) = 0.010 , where Z
is a standard normal random variable.
( )
7. P χ 22 ≥ 9.21 = 0.01, (
P χ 22 ≥ 0.02 = 0.99, ) ( )
P χ 32 ≥ 11.34 = 0.01, ( )
P χ 42 ≥ 9.49 = 0.05,
P(χ 2
4 ≥ 0.71) = 0.95 , P ( χ ≥ 11.07 ) = 0.05
2
5 ( ) ( )
and P χ 52 ≥ 1.15 = 0.95, where P χ n2 ≥ c = α , where χ n2
has a Chi-square distribution with n degrees of freedom.
8. n ! denotes the factorial of n.
9. The determinant of a square matrix A is denoted by | A | .
10. R: The set of all real numbers.
11. R”: n-dimensional Euclidean space.
12. y′ and y′′ denote the first and second derivatives respectively of the function y ( x) with respect to x.

NOTE: This Question-cum-Answer book contains THREE sections, the Compulsory Section A, and
the Optional Sections B and C.
• Attempt ALL questions in the compulsory section A. It has 15 objective type questions of six
marks each and also nine subjective type questions of fifteen marks each.
• Optional Sections B and C have five subjective type questions of fifteen marks each.
• Candidates seeking admission to either of the two programmes, M.Sc. in Applied Statistics &
Informatics at IIT Bombay and M.Sc. in Statistics & Informatics at IIT Kharagpur, are required to
attempt ONLY Section B (Mathematics) from the Optional Sections.
• Candidates seeking admission to the programme, M.Sc. in Statistics at IIT Kanpur, are required
to attempt ONLY Section C (Statistics) from the Optional Sections.
You must therefore attempt either Optional Section B or Optional Section C depending upon the
programme(s) you are seeking admission to, and accordingly tick one of the boxes given below.
B
Optional Section Attempted
C
• The negative marks for the Objective type questions will be carried over to the total marks.
• Write the answers to the objective questions in the Answer Table for Objective Questions
provided on page MS 11/63 only.

1
 Compulsory Section A

( an )
1 n
1. If an > 0 for n ≥ 1 and lim = L < 1, then which of the following series is not convergent?
n→∞
∞
(A) ∑
n =1
an an +1
∞
(B) ∑a
n =1
2
n

∞
(C) ∑
n =1
an
∞
1
(D) ∑
n =1 an

2. Let E and F be two mutually disjoint events. Further, let E and F be independent of G. If
p = P ( E ) + P ( F ) and q = P (G ) , then P ( E ∪ F ∪ G ) is
(A) 1 − pq
(B) q + p 2
(C) p + q 2
(D) p + q − pq

3. Let X be a continuous random variable with the probability density function symmetric about 0. If
V ( X ) < ∞, then which of the following statements is true?
(A) E (| X |) = E ( X )
(B) V (| X |) = V ( X )
(C) V (| X |) < V ( X )
(D) V (| X |) > V ( X )

4. Let
f ( x) = x | x | + | x − 1|, − ∞ < x < ∞.
Which of the following statements is true?
(A) f is not differentiable at x = 0 and x = 1.
(B) f is differentiable at x = 0 but not differentiable at x = 1.
(C) f is not differentiable at x = 0 but differentiable at x = 1.
(D) f is differentiable at x = 0 and x = 1.

5. Let A x = b be a non-homogeneous system of linear equations. The augmented matrix [ A : b ] is given by

% % %
⎡ 1 1 −2 1 1⎤
⎢ −1 2 3 −1 0 ⎥⎥ .
⎢
⎢⎣ 0 3 1 0 −1⎥⎦

2
 Which of the following statements is true?
(A) Rank of A is 3.
(B) The system has no solution.
(C) The system has unique solution.
(D) The system has infinite number of solutions.

6. An archer makes 10 independent attempts at a target and his probability of hitting the target at each attempt
5
is . Then the conditional probability that his last two attempts are successful given that he has a total of 7
6
successful attempts is
1
(A) 5
5
7
(B)
15
25
(C)
36
7 3
8! ⎛ 5 ⎞ ⎛ 1 ⎞
(D) ⎜ ⎟ ⎜ ⎟
3! 5! ⎝ 6 ⎠ ⎝ 6 ⎠

7. Let
f ( x) = ( x − 1)( x − 2 )( x − 3)( x − 4 )( x − 5 ) , − ∞ < x < ∞.
d
The number of distinct real roots of the equation f ( x) = 0 is exactly
dx
(A) 2 (B) 3 (C) 4 (D) 5

8. Let
k | x|
f ( x) = , − ∞ < x < ∞.
(1 + | x |)
4

Then the value of k for which f ( x) is a probability density function is

1
(A)
6
1
(B)
2
(C) 3
(D) 6

M X ( t ) = e 3t +8t
2
9. If is the moment generating function of a random variable X , then
P ( − 4.84 < X ≤ 9.60 ) is
(A) equal to 0.700
(B) equal to 0.925
(C) equal to 0.975
(D) greater than 0.999

3
10. Let X be a binomial random variable with parameters n and p, where n is a positive integer and
( )
0 ≤ p ≤ 1. If α = P | X − np | ≥ n , then which of the following statements holds true for all n and
p?
1
(A) 0 ≤ α ≤
4
1 1
(B) < α ≤
4 2
1 3
(C) < α <
2 4
3
(D) ≤α ≤1
4

11. Let X 1 , X 2 ,..., X n be a random sample from a Bernoulli distribution with parameter p; 0 ≤ p ≤ 1. The
n
n + 2∑ X i
i =1
bias of the estimator for estimating p is equal to
(
2 n+ n )
1 ⎛ 1⎞
(A) ⎜p− ⎟
n +1 ⎝ 2⎠
1 ⎛1 ⎞
(B) ⎜ − p⎟
n+ n ⎝2 ⎠
1 ⎛1 p ⎞
(C) ⎜2 + ⎟−p
n +1 ⎝ n⎠
1 ⎛1 ⎞
(D) ⎜ − p⎟
n +1 ⎝2 ⎠

12. Let the joint probability density function of X and Y be

⎧e − x , if 0 ≤ y ≤ x < ∞,
f ( x, y ) = ⎨
⎩0, otherwise.
Then E ( X ) is
(A) 0.5
(B) 1
(C) 2
(D) 6

4
13. Let f : → be defined as
⎧ tan t
⎪ , t ≠ 0,
f (t ) = ⎨ t
⎪⎩ 1, t = 0.
x3
1
Then the value of lim 2
x →0 x ∫ f ( t ) dt
x2
(A) is equal to −1
(B) is equal to 0
(C) is equal to 1
(D) does not exist

14. Let X and Y have the joint probability mass function;

x
1 ⎛ 2 y +1 ⎞
P ( X = x, Y = y ) = y + 2 ⎜ ⎟ , x, y = 0,1, 2,... .
2 ( y + 1) ⎝ 2 y + 2 ⎠
Then the marginal distribution of Y is
1
(A) Poisson with parameter λ =
4
1
(B) Poisson with parameter λ =
2
1
(C) Geometric with parameter p =
4
1
(D) Geometric with parameter p =
2

1 3
15. Let X 1 , X 2 and X 3 be a random sample from a N ( 3, 12 ) distribution. If X = ∑ X i and
3 i =1
1 3
∑ ( Xi − X )
2
S2 = denote the sample mean and the sample variance respectively, then
2 i =1
(
P 1.65 < X ≤ 4.35, 0.12 < S 2 ≤ 55.26 is )
(A) 0.49
(B) 0.50
(C) 0.98
(D) none of the above

5
 16. (a) Let X 1 , X 2 , ... , X n be a random sample from an exponential distribution with the probability density
function;
⎧⎪θ e−θ x , if x > 0,
f (x;θ ) = ⎨
⎪⎩0, otherwise,
where θ > 0. Obtain the maximum likelihood estimator of P ( X > 10 ) . 9 Marks
(b) Let X 1 , X 2 , ... , X n be a random sample from a discrete distribution with the probability mass function
given by
1−θ 1 θ
P ( X = 0) = ; P ( X = 1) = ; P ( X = 2 ) = , 0 ≤ θ ≤ 1.
2 2 2
Find the method of moments estimator for θ . 6 Marks

17. (a) Let A be a non-singular matrix of order n (n > 1), with | A | = k . If adj ( A) denotes the adjoint of the
matrix A , find the value of | adj ( A) | . 6 Marks
(b) Determine the values of a, b and c so that (1, 0, − 1) and ( 0, 1, − 1) are eigenvectors of the matrix,
⎡2 1 1⎤
⎢a 3 2⎥ . 9 Marks
⎢ ⎥
⎢⎣ 3 b c ⎥⎦

18. (a) Using Lagrange’s mean value theorem, prove that

b−a b−a
< tan −1 b − tan −1 a < ,
1+ b 2
1 + a2
π
where 0 < tan −1 a < tan −1 b < . 6 Marks
2
(b) Find the area of the region in the first quadrant that is bounded by y = x , y = x − 2 and the x − axis .
9 Marks
19. Let X and Y have the joint probability density function;
⎧ − ( x2 + 2 y 2 )
⎪ c x y e , if x > 0, y > 0,
⎪
f ( x, y ) = ⎨
⎪0, otherwise.
⎪⎩
(
Evaluate the constant c and P X 2 > Y 2 . )
20. Let PQ be a line segment of length β and midpoint R. A point S is chosen at random on PQ. Let X ,
the distance from S to P, be a random variable having the uniform distribution on the interval ( 0, β ) .
Find the probability that PS , QS and PR form the sides of a triangle.

21. Let X 1 , X 2 , ... , X n be a random sample from a N ( μ , 1) distribution. For testing H 0 : μ = 10 against
n
1
H1 : μ = 11, the most powerful critical region is X ≥ k , where X =
n
∑
i =1
X i . Find k in terms of n such

that the size of this test is 0.05.

Further determine the minimum sample size n so that the power of this test is at least 0.95.
6
22. Consider the sequence {sn } , n ≥ 1, of positive real numbers satisfying the recurrence relation
sn −1 + sn = 2 sn +1 for all n ≥ 2 .
1
(a) Show that | sn +1 − sn | = | s2 − s1 | for all n ≥ 1 .
2n −1
(b) Prove that {sn } is a convergent sequence.

23. The cumulative distribution function of a random variable X is given by

⎧0, if x < 0,
⎪1
⎪ 1 + x3 ,
⎪5
( ) if 0 ≤ x < 1,
F ( x) = ⎨
⎪ 1 ⎡3 + ( x − 1)2 ⎤ , if 1 ≤ x < 2,
⎪5 ⎣ ⎦
⎪1, if x ≥ 2.
⎩
⎛1 3⎞
Find P ( 0 < X < 2 ) , P ( 0 ≤ X ≤ 1) and P ⎜ ≤ X ≤ ⎟ .
⎝2 2⎠

( )
24. Let A and B be two events with P ( A | B ) = 0.3 and P A | B c = 0.4 . Find P( B | A) and P( B c | Ac ) in
1 1 1 9
terms of P( B). If ≤ P( B | A) ≤ and ≤ P( B c | Ac ) ≤ , then determine the value of P( B).
4 3 4 16

Optional Section B
25. Solve the initial value problem
( )
y′ − y + y 2 x 2 + 2 x + 1 = 0, y (0) = 1.
26. Let y1 ( x) and y2 ( x) be the linearly independent solutions of
x y′′ + 2 y′ + x e x y = 0.
If W ( x) = y1 ( x) y2′ ( x) − y2 ( x) y1′( x) with W (1) = 2, find W (5).

1 1

∫0 ∫y x e x y dx dy.
2
27. (a) Evaluate 9 Marks

(b) Evaluate ∫∫∫

W
z dx dy dz , where W is the region bounded by the planes x = 0, y = 0, z = 0, z = 1

and the cylinder x 2 + y 2 = 1 with x ≥ 0, y ≥ 0.

6 Marks
28. A linear transformation T : → 3 2
is given by
T ( x, y, z ) = ( 3 x + 11 y + 5 z , x + 8 y + 3 z ) .
Determine the matrix representation of this transformation relative to the ordered bases
{(1, 0, 1) , ( 0, 1, 1) , (1, 0, 0 )} , {(1, 1) , (1, 0 )} . Also find the dimension of the null space of this transformation.

7
 ⎧ x2 + y2
⎪ , if x + y ≠ 0,
29. (a) Let f ( x, y ) = ⎨ x + y
⎪0, if x + y = 0.
⎩
Determine if f is continuous at the point ( 0, 0 ) . 6 Marks
(b) Find the minimum distance from the point (1, 2, 0 ) to the cone z = x + y . 2 2 2
9 Marks

Optional Section C

30. Let X 1 , X 2 , ... , X n be a random sample from an exponential distribution with the probability density
function;
⎧ 1 − θx
⎪ e , if x > 0,
f ( x ; θ ) = ⎨θ
⎪0,
⎩ otherwise,
where θ > 0. Derive the Cramér-Rao lower bound for the variance of any unbiased estimator of θ .
1 n
Hence, prove that T = ∑ X i is the uniformly minimum variance unbiased estimator of θ .
n i =1

31. Let X 1 , X 2 , ... be a sequence of independently and identically distributed random variables with the
probability density function;
⎧1 2 − x
⎪ x e , if x > 0,
f ( x) = ⎨ 2
⎪⎩0, otherwise.

( (
Show that lim P X 1 + ... + X n ≥ 3 n − n ≥ .
n→∞
))1
2

32. Let X 1 , X 2 , ... , X n be a random sample from a N μ , σ 2 ( ) distribution, where both μ and σ 2 are
unknown. Find the value of b that minimizes the mean squared error of the estimator
n 2 n

∑ (X i − X ) for estimating σ , where X =

b 1
Tb =
n −1 i =1
2

n
∑X .
i =1
i

( )
33. Let X 1 , X 2 , ... , X 5 be a random sample from a N 2, σ 2 distribution, where σ 2 is unknown. Derive the
most powerful test of size α = 0.05 for testing H 0 : σ = 4 against H1 : σ 2 = 1.
2

34. Let X 1 , X 2 , ... , X n be a random sample from a continuous distribution with the probability density
function;
⎧ 2 x − x2
⎪ λ , if x > 0,
f (x; λ) = ⎨ λ e
⎪0,
⎩ otherwise,
where λ > 0. Find the maximum likelihood estimator of λ and show that it is sufficient and an unbiased
estimator of λ .
8