ECMT2150 INTERMEDIATE ECONOMETRICS Week 2 Tutorial
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
ECMT2150 INTERMEDIATE ECONOMETRICS
Week 2 Tutorial
OLS, Properties of OLS, Start using Stata
1. (Wooldridge Question 2.2) In the simple linear regression model = 0 + 1 + suppose that () ≠ 0. Letting 0 = (), show that the model can always be written with the same slope, but a new intercept and error, where the new error has a zero expected value.
2. Which of the following models are (or can be transformed into) linear regression models?
a. = 0 + 1 +
b. = 0 + 1 ln +
c. d. e.
f.
ln = 0 + 1 +
= 0 exp(1 + )
= 0 + +
= 0 + 1 (1⁄ ) +
3. (adapted from Wooldridge Question 3.5) In a study relating marks obtained by
students in undergraduate econometrics (metrics) in Australian universities to time spent in various activities, a survey is conducted among several students. The students are given questionnaires and asked to write down how many hours they spend each week in four activities: studying, sleeping, working, and leisure. Any activity is put into one of the four categories, so that for each student, the sum of hours in the four activities must be 168.
a. In the model
= 0 + 1 + 2 + 3 + 4 + does it make sense to hold sleep, work, and leisure fixed, while changing study?
b. Explain why this model violates assumption MLR.3.
c. How could you reformulate the model so that its parameters have a useful interpretation and it satisfies assumption MLR.3?
4. For each of the following, state whether it can cause OLS estimators to be biased?
a. Heteroskedasticity.
b. Omitting an important variable.
c. A sample correlation coefficient of .95 between two independent variables both included in the model.
For each, if your answer is no, then say why it does not cause bias in the OLS estimator. If yes, explain the source of the bias.
5. (Computer Exercise) The data file hprice1 (hprice1.dta) contains a small sample of house prices. Use this dataset to examine the relationship between house prices, lot (property) sizes, house sizes, and the number of bedrooms.
a. First, begin by reporting the average, minimum and maximum values, and the standard deviation for the house price (in $000s) and the number of bedrooms.
b. Now, consider the following model:
y = β0 +β1x1 +β2x2 +β3x3 +u,
wherey is the house price (in $000’s), x1 the number of bedrooms, x2 the lot size
(in square feet), and x3 the house size (in square feet).
i. Estimate this model using OLS.
ii. Interpret your coefficient estimates (j,j = 0, 1, 2, 3).
(c) Now, consider instead the following related model: lny = β0 +β1x1 +β2lnx2 +β3lnx3 +u,
wherey, x1, x2, andx3 are defined as above.
i. Estimate this model using OLS.
ii. Howwould you interpret your coefficient estimates now?
(d) Now, modify your model of (3c) by including the (natural log of) the assessed value of the house in the model. In particular, consider the model:
lny = β0 +β1x1 +β2lnx2 +β3lnx3 +β4lnx4 +u,
wherex4 is the assessed value of the house (in $000’s) and the othervariables are
as before.
i. Estimate this model using OLS.
ii. Interpret the coefficients of your model.
iii. What impact has the introduction of the assessed value variable had on the estimated coefficients, 1, 2, and 3? Can you explain this change?
iv. How would you describe the causal relationship betweeny andx4?
6. (Wooldridge Question 2.7) Consider the savings function:
= 0 + 1 + , = ∙ ,
where is a random variable with () = 0 and () = . Assume that e is independent of inc.
Show that E( |) = 0, so that the key zero conditional mean assumption (Assumption SLR.4) is satisfied. [Hint: If is independent of , then E( |) = E(). ]
Show that ( |) = , so that the homoscedasticity Assumption SLR.5 is violated. In particular, the variance of sav increases with inc. [Hint: ( |) = () if and are independent.]
Provide a discussion that supports the assumption that the variance of savings increases with family income.
Extra questions if you are wanting to do more:
1. Suppose someone has given you the following regression results:
yˆt =2.6911 −0.4795xt
wherey is the coffee consumption in Australia (cups per person per day); x is the retail price of coffee ($ per kilo); and t is the time period.
[Let us assume for simplicity that this is a demand curve. Note that demand and supply side factors will jointly determine the relationship between price and quantity, so estimating a demand equation can be complicated.]
a. What is the interpretation of the intercept in this example? Does it make economic sense?
b. Howwould you interpret the slope coefficient?
c. Is it possible to tell what the true least squares line is? That is, can you find β0 and β1?
d. The price elasticity of demand is defined as the percentage change in the quantity demanded for a percentage change in the price. That is, the
elasticity of y with respect to x is defined as = . Note that is just the
slope of y with respect to x. From the above regression results, can you determine the elasticity of demand for coffee? If not, what additional information do you need?
2. (Computer Exercise) Use the data in WAGE2 to estimate a simple regression explaining monthly salary (wage) in terms of IQ score (IQ). IQ (intelligence quotient) tests were developed over 100 years ago and attempt to measure a person’s innate cognitive ability (IQ tests are sometimes referred to as tests of ‘general intelligence’). There is a substantial body of research which examines whether IQ is related to a range of outcomes such as occupational status, income and even criminal activity. In this exercise we consider whether and how IQ affect the wage people earn in the labour market.
a. Report the average, minimum and maximum values, and the standard deviation for wage, education and IQ in the sample (IQ scores are standardized so that the average in the population is 100 with a standard deviation equal to 15).
b. Estimate a simple regression model where a one-point increase in IQ changes wage by a constant dollar amount. Use this model to find the predicted increase in wage for an increase in IQ of 15 points. Does IQ explain most of the variation in wage?
c. Now, estimate a model where each one-point increase in IQ has the same percentage effect on wage. If IQ increases by 15 points, what is the approximate percentage increase in predicted wage?
d. Do you think the simple regression captures a causal effect of IQ on the wage? Explain.
3. (Wooldridge Question 3.4) The median starting salary for new law school graduates is determined by:
log() = 0 + 1 + 2 + 3 log() + 4 log() + 5 + ,
where is the median LSAT score for the graduation class, is the median college GPA for the class, is the number of volumes in the law school library, is the annual cost of attending law school, and is a law school ranking (with = 1 being the best).
Explain why we expect 5 ≤ 0.
What signs do you expect for the other slope parameters? Justify your answers.
Using the data in LAWSCH86 (you do not need to do any regression), the estimated equation is
log() = 8.34 + .0047 + .248 + .095 log() +.038 log() − .0033
= 136, 2 = .842
What is the predicted ceteris paribus difference in salary for schools with a median GPA different by one point? (Report your answer as a percentage.) Interpret the coefficient on the variable log().
Would you say it is better to attend a higher ranked law school? How much is a difference in ranking of 20 worth in terms of predicted starting salary?
2022-03-07
OLS, Properties of OLS, Start using Stata