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R代写 - STAT 244 Midterm

时间：2020-12-03

1. Please write the following statement “”I have not given or received any unauthorized assistance on

this examination” and sign it. Your work will not be graded without such a signed statement.

2. For the Calculations questions, be sure to show your computations and to write down whatever

equation you use. In order to receive full credit for a problem, you should show all of your work

and explain your reasoning. Circle your final answer. Good work can receive substantial partial

credit even if the final answer is incorrect.

3. If you make a mistake in a multi-part question, later parts will be graded for following through

correctly. You should try all parts of a problem even if you get stuck on an early part. If necessary,

state and assume a value from an earlier part.

4. If you are unsure of what a question is asking for, do not hesitate to ask the instructor for clarification

via email.

5. You may need to use a calculator to answer some questions.

Points Value

Multiple Choice 20

Question 5 16

Question 6 16

Question 7 12

Question 8 16

Question 9 10

Question 10 10

TOTAL 100

1 Multiple Choice (20 points)

Do not explain your answers for this section—just circle one.

1. [5 pts] Let A and B be events, which each have nonzero probability. Suppose that A and B are

disjoint. Are A and B independent?

(a) Yes. With the information given, we can conclude that A and B are independent.

(b) No. With the information given, we can conclude that A and B are not independent.

(c) Unknown. There is not enough information given to determine whether A and B are indepen-

dent or not.

.

2. [5 pts] For independent random variables X and Y

Var(X − Y ) = Var(X)− Var(Y )

is:

(a) True

(b) False

.

3. [5 pts] For a continuous random variable X with density f(x),

E(log(X)) =

∫ ∞

x=−∞

log(x) · f(log(x)) dx.

(a) True

(b) False

.

4. [5 pts] Suppose we draw 10 cards from a standard deck, without replacement. Then the distribution

of X, the number of Kings that we have, is Binomial(10, 4/52).

(a) True

(b) False

(Note—you do not need to numerically calculate the correlation/covariance to answer this question.)

.

2 Calculations (80 points)

For full credit, show your work for all problems in this section.

5. [16 pts] Suppose I roll a die 10 times. Let X = # of 1’s and Y = # of 2’s.

(a) [8 pts] Calculate P(X = 5 | Y = 5).

(b) [8 pts]Calculate E(XY )

6. [16 pts] Let X, Y both be Exponential(2) random variables, X, Y are independent. This distribu-

tion has density fX(x) = 2e

−2x, fY (y) = 2e−2y and CDF FX(x) = 1 − e−2x, FY (y) = 1 − e−2y on

the support x, y ∈ (0,∞).

(a) [6 pts] Find the PDF of X − Y .

(b) [6 pts] Let Z = bXc, which is X rounded down to an integer. For example if X = 2.1 then

Z = 2, or if X = 5.9 then Z = 5. Find the CDF of Y − Z. You can have answers with

summation/integral notations.

(c) [4 pts] Let D = X − Z, the decimal part of X. For example if X = 2.1 then Z = 2 and

D = 0.1. Are Z and D independent or not? Explain.

7. [12 pts] Consider the following procedure. We have a fair coin. First, we flip the coin once. Let

Y = the outcome of the first flip (Y = 1 if Heads, Y = 0 if Tails). If the first flip comes up Heads,

we then flip the coin two additional times; if the first flip comes up Tails, then we instead flip the

coin four additional times. Let Z be the number of Heads in these additional flips (not counting the

initial first flip).

(a) [6 pts] Calculate E(Z).

(b) [6 pts] Calculate P(Y = 0 | Z = 0).

8. [16 pts] Let X and Z be independent draws from a Uniform[−1, 1] distribution.

(a) [6 pts] Let Y = 1 − X2. Find either the CDF of Y , FY (y), or the density of Y , fY (y), and

also state the support of Y , i.e. the range of possible values. You do not need to compute both

the CDF and the density.

(b) [4 pts] Multiple choice—what is the correlation, Corr(X,Y )? Circle one:

i. Equal to −1.

ii. Negative, but not −1.

iii. Equal to 0.

iv. Positive, but not 1.

v. Equal to 1.

You do not need to calculate the correlation, but you may briefly explain your answer (not

required).

(c) [6 pts] Let A = (X +Z)2 and calculate E(A). You may use the following facts: a Uniform[0,1]

random variable U has E(U) = 12 and Var(U) =

1

12 .

9. [10 pts] Let (X,Y ) be random variables supported on the unit square [0, 1]×[0, 1], with joint density

f(x, y) = 0.5 + 2xy. Calculate the conditional density fY |X(y | x).

10. [10 pts] The unit interval [0, 1] is broken into two pieces, with the break point chosen uniformly

at random along the interval. Next, a point X on the unit interval [0, 1] is chosen uniformly at

random. What is the probability that X is in the larger of the two pieces? It’s fine to have integral

or summation notation in your final answer, but no probability/statistics notation such as pX or

E(X) etc.