ECON120A: Homework 2
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ECON120A: Homework 2
Exercise 1. [5 points] Warming up
A bank classifies borrowers as high risk or low risk. Only 15% of its loans are made to those in the high-risk category. Of all its loans, 5% are in default, and 40% of those in default were made to high-risk borrowers. What is the probability that a high-risk borrower will default?
Exercise 2. [20 points] Expectation and Variance
a) Use linearity of the expectation and the definition of population variance to show that, for a random variable X with finite moments: [10 points]
E[X2] = (E[X])2 + V[X]
b) Use linearity of expectation and the definition of covariance to show that:
Cov(aX + bY, cZ + dW) = ac . Cov(X, Z) + ad . Cov(X, W) + bc . Cov(Y, Z) + bd . Cov(Y, W)
where X, Y, Z, W are random variables and a, b, c, d are real numbers. We call this property bilinearity, that is Cov(., .) is linear in each of its arguments. [10 points]
Exercise 3. [25 points] Sample Space and Independence
A graduating engineer has signed up for three job interviews. She intends to categorize each one as being either a “success” or a “failure” depending on whether it leads to a plant trip.
a) Write out the appropriate sample space for the three job interviews “random experiment.” [5 points]
b) How many numbers do you need to know to completely specify this probability distribution? Remem- ber, it is enough to specify what is the probability of each point in the sample space. [5 points]
c) How does your answer to part b) change if you learn that each internship outcome is independent of all others? [5 points]
d) How does your answer to part b) change if you learn that each internship outcome is independent of all others and that the three internship outcomes follow the same distribution? [5 points]
e) Give your reasoning on why you believe it is unrealistic to assume that the interview outcomes are independent. [5 points]
Exercise 4. [15 points] Standardization
You are given a random variable X, which represents the age of the candidates for a senior position in a tech company. You know E[X] = µ and V(X) = σ 2 are finite.
a) Consider the random variable Y = . Compute its expectation E[Y] and its V(Y). [10 points]
b) Now suppose we additionally know that X is distributed N(45, 5). Use Y and the normal tables/cal-
culator to compute P (40 < X < 43). [5 points]
Exercise 5. [15 points] Conditional Probabilities and Floods in Jakarta
Jakarta, the capital city of Indonesia, has historically experienced a high risk of floods. Some developers have created an app to evaluate the risk of a flood in fragile areas given the rainfall observed in a nearby location. Denote F to be the event that a flood occurs. R denotes a continuous random variable, weekly rainfall measured in millimeters. You are given the following joint frequency table.
|
< 1 |
[1, 5) |
[5, 10) |
[10, 20) |
[20, 50) |
[50, 100) |
100+ |
Total |
Flood |
0.01 |
0.01 |
0.02 |
0.07 |
0.12 |
0.07 |
0.05 |
0.35 |
No Flood |
0.05 |
0.14 |
0.23 |
0.14 |
0.06 |
0.02 |
0.01 |
0.65 |
Total |
0.06 |
0.15 |
0.25 |
0.21 |
0.18 |
0.09 |
0.06 |
1 |
Table 1: Rainfall & Flood Risk in Jakarta
a) Compute the conditional probability of flood for each class of rainfall intensity. Fill in the following table. [5 points]
Rainfall intensity
< 1
[1, 5)
[5, 10)
[10, 20)
[20, 50)
[50, 100)
100+
b) Compute the probability of a flood given that the rainfall is below 50mm. That is compute P (F IR < 50mm). [5 points]
c) Compute the probability of no flood occurring when the rain fall is above 50mm. That is, compute P (Fc IR > 50mm). [5 points]
Exercise 6. [20 points] Conditional Expectation
You are offered the following game:
-A 6-faced fair die is rolled. Call the result of this roll J
-A coin is flipped, if it lands heads you win 2J dollars, if it lands tails you win J.
a) What is the probability you win more than 25$ at this game? [10 points]
b) Compute the conditional expectation of the amount you win conditional on J and then use the Law of Iterated Expectations to compute the expected amount you win after playing the game. [10 points]
2022-08-17