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# ISI MStat Entrance 2020 Problems and Solutions PSA & PSB This post contains ISI MStat Entrance PSA and PSB 2020 Problems and Solutions that can be very helpful and resourceful for your ISI MStat Preparation.

## ISI MStat Entrance 2020 Problems and Solutions - Subjective Paper

### ISI MStat 2020 Problem 1

Let f(x)=x2−2x+2. Let L1 and L2 be the tangents to its graph at x=0 and x=2 respectively. Find the area of the region enclosed by the graph of f and the two lines L1 and L2.

### ISI MStat 2020 Problem 2

Find the number of 3×3 matrices A such that the entries of A belong to the set Z of all integers, and such that the trace of AtA is 6 . (At denotes the transpose of the matrix A).

Solution

### ISI MStat 2020 Problem 3

Consider $n$ independent and identically distributed positive random variables $X_{1}, X_{2}, \ldots, X_{n}$. Suppose $S$ is a fixed subect of ${1,2, \ldots, n}$ consisting of $k$ distinct ekements where $1 \leq k<n$.
(a) Compute
$$\mathrm{E}\left[\frac{\sum_{i \in s} X_{i}}{\sum_{i=1}^{\infty} X_{i}}\right]$$
(b) Assume that $X_{i}$ is have mean $\mu$ and variance $\sigma^{2}, 0<\sigma^{2}<\infty$. If $j \notin S$, show that the correlation between ( $\left.\sum_{i \in s} X_{i}\right) X_{j}$ and $\sum_{i \in}X_{i}$ lies between $-\frac{1}{\sqrt{k+1}}$ and $\frac{1}{\sqrt{k+1}}$.

Solution

### ISI MStat 2020 Problem 4

Let X1,X2,…,Xn be independent and identically distributed random variables. Let Sn=X1+⋯+Xn. For each of the following statements, determine whether they are true or false. Give reasons in each case.

(a) If Sn∼Exp with mean n, then each Xi∼Exp with mean 1 .

(b) If Sn∼Bin(nk,p), then each Xi∼Bin(k,p)

Solution

### ISI MStat 2020 Problem 5

Let U1,U2,…,Un be independent and identically distributed random variables each having a uniform distribution on (0,1) . Let X=min{U1,U2,…,Un}, Y=max{U1,U2,…,Un}

Evaluate E[X∣Y=y] and E[Y∣X=x].

Solution

### ISI MStat 2020 Problem 6

Suppose individuals are classified into three categories C1,C2 and C3 Let p2,(1−p)2 and 2p(1−p) be the respective population proportions, where p∈(0,1). A random sample of N individuals is selected from the population and the category of each selected individual recorded.

For i=1,2,3, let Xi denote the number of individuals in the sample belonging to category Ci. Define U=X1+X32

(b) Show that the mean squared error of UN is p(1−p)2N

Solution

### ISI MStat 2020 Problem 7

Consider the following model:
$$y_{i}=\beta x_{i}+\varepsilon_{i} x_{i}, \quad i=1,2, \ldots, n$$
where $y_{i}, i=1,2, \ldots, n$ are observed; $x_{i}, i=1,2, \ldots, n$ are known positive constants and $\beta$ is an unknown parameter. The errors $\varepsilon_{1}, \varepsilon_{2}, \ldots, \varepsilon_{n}$ are independent and identically distributed random variables having the
probability density function
$$f(u)=\frac{1}{2 \lambda} \exp \left(-\frac{|u|}{\lambda}\right),-\infty<u<\infty$$
and $\lambda$ is an unknown parameter.
(a) Find the least squares estimator of $\beta$.
(b) Find the maximum likelihood estimator of $\beta$.

Solution

### ISI MStat 2020 Problem 8

Assume that $X_{1}, \ldots, X_{n}$ is a random sample from $N(\mu, 1)$, with $\mu \in \mathbb{R}$. We want to test $H_{0}: \underline{\mu}=0$ against $H_{1}: \mu=1$. For a fixed integer $m \in{1, \ldots, n}$, the following statistics are defined:

\begin{aligned}
T_{1} &=\left(X_{1}+\ldots+X_{m}\right) / m \\
T_{2} &=\left(X_{2}+\ldots+X_{m+1}\right) / m \\
\vdots &=\vdots \\
T_{n-m+1} &=\left(X_{n-m+1}+\ldots+X_{n}\right) / m .
\end{aligned}

Fix $\alpha \in(0,1)$. Consider the test

reject $H_{0}$ if max {${T_{i}: 1 \leq i \leq n-m+1}>c_{m, \alpha}$}

Find a choice of $c_{m, \alpha}$ $\mathbb{R}$ in terms of the standard normal distribution
function $\Phi$ that ensures that the size of the test is at most $\alpha$.

Solution

### ISI MStat 2020 Problem 9

• A finite population has N units, with xi being the value associated with the i th unit, i=1,2,…,N. Let x¯N be the population mean. A statistician carries out the following experiment.

Step 1: Draw an SRSWOR of size n(1 and denote the sample mean by X¯n

Step 2: Draw an SRSWR of size m from S1. The x -values of the sampled units are denoted by {Y1,…,Ym}

An estimator of the population mean is defined as,

Tˆm=1m∑i=1mYi

(a) Show that Tˆm is an unbiased estimator of the population mean.

(b) Which of the following has lower variance: Tˆm or X¯n?

Solution

## ISI MStat 2020 - Objective Paper

### ISI MStat 2020 PSA Answer Key

#### ISI MStat 2020 Probability Problems Discussion [Recorded Class]

This post contains ISI MStat Entrance PSA and PSB 2020 Problems and Solutions that can be very helpful and resourceful for your ISI MStat Preparation.

## ISI MStat Entrance 2020 Problems and Solutions - Subjective Paper

### ISI MStat 2020 Problem 1

Let f(x)=x2−2x+2. Let L1 and L2 be the tangents to its graph at x=0 and x=2 respectively. Find the area of the region enclosed by the graph of f and the two lines L1 and L2.

### ISI MStat 2020 Problem 2

Find the number of 3×3 matrices A such that the entries of A belong to the set Z of all integers, and such that the trace of AtA is 6 . (At denotes the transpose of the matrix A).

Solution

### ISI MStat 2020 Problem 3

Consider $n$ independent and identically distributed positive random variables $X_{1}, X_{2}, \ldots, X_{n}$. Suppose $S$ is a fixed subect of ${1,2, \ldots, n}$ consisting of $k$ distinct ekements where $1 \leq k<n$.
(a) Compute
$$\mathrm{E}\left[\frac{\sum_{i \in s} X_{i}}{\sum_{i=1}^{\infty} X_{i}}\right]$$
(b) Assume that $X_{i}$ is have mean $\mu$ and variance $\sigma^{2}, 0<\sigma^{2}<\infty$. If $j \notin S$, show that the correlation between ( $\left.\sum_{i \in s} X_{i}\right) X_{j}$ and $\sum_{i \in}X_{i}$ lies between $-\frac{1}{\sqrt{k+1}}$ and $\frac{1}{\sqrt{k+1}}$.

Solution

### ISI MStat 2020 Problem 4

Let X1,X2,…,Xn be independent and identically distributed random variables. Let Sn=X1+⋯+Xn. For each of the following statements, determine whether they are true or false. Give reasons in each case.

(a) If Sn∼Exp with mean n, then each Xi∼Exp with mean 1 .

(b) If Sn∼Bin(nk,p), then each Xi∼Bin(k,p)

Solution

### ISI MStat 2020 Problem 5

Let U1,U2,…,Un be independent and identically distributed random variables each having a uniform distribution on (0,1) . Let X=min{U1,U2,…,Un}, Y=max{U1,U2,…,Un}

Evaluate E[X∣Y=y] and E[Y∣X=x].

Solution

### ISI MStat 2020 Problem 6

Suppose individuals are classified into three categories C1,C2 and C3 Let p2,(1−p)2 and 2p(1−p) be the respective population proportions, where p∈(0,1). A random sample of N individuals is selected from the population and the category of each selected individual recorded.

For i=1,2,3, let Xi denote the number of individuals in the sample belonging to category Ci. Define U=X1+X32

(b) Show that the mean squared error of UN is p(1−p)2N

Solution

### ISI MStat 2020 Problem 7

Consider the following model:
$$y_{i}=\beta x_{i}+\varepsilon_{i} x_{i}, \quad i=1,2, \ldots, n$$
where $y_{i}, i=1,2, \ldots, n$ are observed; $x_{i}, i=1,2, \ldots, n$ are known positive constants and $\beta$ is an unknown parameter. The errors $\varepsilon_{1}, \varepsilon_{2}, \ldots, \varepsilon_{n}$ are independent and identically distributed random variables having the
probability density function
$$f(u)=\frac{1}{2 \lambda} \exp \left(-\frac{|u|}{\lambda}\right),-\infty<u<\infty$$
and $\lambda$ is an unknown parameter.
(a) Find the least squares estimator of $\beta$.
(b) Find the maximum likelihood estimator of $\beta$.

Solution

### ISI MStat 2020 Problem 8

Assume that $X_{1}, \ldots, X_{n}$ is a random sample from $N(\mu, 1)$, with $\mu \in \mathbb{R}$. We want to test $H_{0}: \underline{\mu}=0$ against $H_{1}: \mu=1$. For a fixed integer $m \in{1, \ldots, n}$, the following statistics are defined:

\begin{aligned}
T_{1} &=\left(X_{1}+\ldots+X_{m}\right) / m \\
T_{2} &=\left(X_{2}+\ldots+X_{m+1}\right) / m \\
\vdots &=\vdots \\
T_{n-m+1} &=\left(X_{n-m+1}+\ldots+X_{n}\right) / m .
\end{aligned}

Fix $\alpha \in(0,1)$. Consider the test

reject $H_{0}$ if max {${T_{i}: 1 \leq i \leq n-m+1}>c_{m, \alpha}$}

Find a choice of $c_{m, \alpha}$ $\mathbb{R}$ in terms of the standard normal distribution
function $\Phi$ that ensures that the size of the test is at most $\alpha$.

Solution

### ISI MStat 2020 Problem 9

• A finite population has N units, with xi being the value associated with the i th unit, i=1,2,…,N. Let x¯N be the population mean. A statistician carries out the following experiment.

Step 1: Draw an SRSWOR of size n(1 and denote the sample mean by X¯n

Step 2: Draw an SRSWR of size m from S1. The x -values of the sampled units are denoted by {Y1,…,Ym}

An estimator of the population mean is defined as,

Tˆm=1m∑i=1mYi

(a) Show that Tˆm is an unbiased estimator of the population mean.

(b) Which of the following has lower variance: Tˆm or X¯n?

Solution

## ISI MStat 2020 - Objective Paper

### ISI MStat 2020 PSA Answer Key

#### ISI MStat 2020 Probability Problems Discussion [Recorded Class]

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### 25 comments on “ISI MStat Entrance 2020 Problems and Solutions PSA & PSB”

1. Partha pratim das says:

What is ans of no. 15?

2. ISHAN PATHAK says:

For Q3: the set of points A,B,C,D for D=(-2,1) do not form a parallelogram
Parallelogram is possible only for options A(4,1) and option B(-2,-3) but since A,B,C,D are in clockwise in same order, it should be option A(4,1)
For Q9 distance between the centres of two circles C1(2,2) and C2(-2,-1) id 5=sum of radii(2+3) so the circles touch extrenally, no. of common tangents should be 3
For Q10 the determinant should be 1, can be verified as A^17=A,A^10=A^2: and A^10+A^10-I={(1,4,10),(0,-1,0),(0,0,-1)} hence det=product of disgonal elements =1 Answer should be A(1)
For Q15, each element of A consists of k ones followed by n−k zeroes, where k∈{0,1,…,n}, hence there are n+1 possible elements (option B)

1. Srijit Mukherjee says:

Thanks, Ishan for such a valuable suggestion and being a student of Cheenta. We will make the changes and will discuss the same in our upcoming classes.

3. Partha pratim das says:

Q.15 pls elaborate a little not getting it

4. Partha pratim das says:

I am getting Q1 to be c but here it is b may be I am wrong it will be helpful if you pls tell the logic

5. Partha pratim das says:

The solution for subjective part is not there. From where can I get it?

6. Prasun says:

Even my answer is coming out to be (c) in Q1

7. blaster says:

what do you expect the cut off to be ?

8. blaster says:

when can we expect the results to come out ?

1. Anton Blaster says:

No answers ? No expected date ?

9. Rintaro Okabe says:

I think the 2020 paper was slightly harder than 2019. So , the cutoff should be lesser this year. Of course , all of this is just my opinion. Anything can happen.

10. Levi Ackerman says:

What is your opinion on the cutoff ?

11. Dexter says:

I think atleast 4 psb questions will suffice.

12. Anton says:

Any info about when the results will come out ?

13. Dexter says:

No idea. Are you expecting a good result?

14. Satou Kazuma says:

@Anton , what is your opinion on exam difficulty ?

15. Anton says:

PSB is a bit difficult than previous year papers for sure.

16. Anton says:

Not sure. Depends on how they grade psb.

17. Anton says:

How did you do ? I think 60+ would be a good score in psb, assuming you score 90+ in psa.

18. Anton says:

How did you do ?

19. Kiyotaka Ayanokoji says:

Hey Anton , how much do you think will be the cutoff for PSA + (2.5)*PSB ? Also , how many marks are you getting in PSA ?

20. B Ban says:

I am getting 105 in PSA and expecting 70-80 in PSB . Is there any chance for me?

21. Dexter says:

This group has been deleted it seems

22. Kamaboko Gonpachiro says:

ISI is extremely late in declaring the result. Don't know what is wrong with them. Did anyone contact them in any way to get some info about results ? Really frustrating...

23. Dexter says:

Any update regarding the result??

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