Comparing private and public college tuition. According to the Chronicle of Higher Education Almanac, 4-year private colleges charge, on average, five times as much for tuition and fees than 4-year public colleges. In order to estimate the true difference in the mean amounts charged for an academic year, random samples of 40 private colleges and 40 public colleges were contacted and questioned about their tuition structures.

  1. Which of the procedures described in Chapter 8 could be used to estimate the difference in mean charges between private and public colleges?

  2. Propose a regression model involving the qualitative independent variable type of college that could be used to investigate the difference between the means. Be sure to specify the coding scheme for the dummy variable in the model.

  3. Explain how the regression model you developed in part b could be used to estimate the difference between the population means.

Short Answer

Expert verified
  1. The method of independent sampling to find the difference between two population means would be used here.

  2. The regression model can be written asEy= β0=β1x1 where x1 denotes the type of college.

The estimated regression model developed in part b can be used to infer conclusions about the population means. When x1 = 1, Ey= β0+β1and when x1 = 0 (meaning private college charges) which is taken as the base level here, E(y) = β0. Therefore, for x1 = 1, Ey= β0+β1denotes the mean difference in the charges between private and public colleges.

Step by step solution

01

Difference between private and public college charges


The method of independent sampling to find the difference between two population means would be used here.

02

Regression model

Here to find a model indicating difference between means of private and public college charges a qualitative variable; x1; to denote the type of college is introduced where

X1= 1, if private college

0, if public college

The regression model can be written as Ey=β0+β1x1

03

Difference between population means

The estimated regression model developed in part b can be used to infer conclusions about the population means.

When x1= 1,Ey= β0+β1 and when x1 = 0 (meaning private college charges) which is taken as the base level here, E (y) = β0

Therefore, for x1 = 1, Ey= β0+β1denotes the mean difference in the charges between private and public colleges.

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Most popular questions from this chapter

Question: Refer to Exercise 12.82.

a. Write a complete second-order model that relates E(y) to the quantitative variable.

b. Add the main effect terms for the qualitative variable (at three levels) to the model of part a.

c. Add terms to the model of part b to allow for interaction between the quantitative and qualitative independent variables.

d. Under what circumstances will the response curves of the model have the same shape but different y-intercepts?

e. Under what circumstances will the response curves of the model be parallel lines?

f. Under what circumstances will the response curves of the model be identical?

Question: Write a regression model relating E(y) to a qualitative independent variable that can assume three levels. Interpret all the terms in the model.

Question: Reality TV and cosmetic surgery. Refer to the Body Image: An International Journal of Research (March 2010) study of the impact of reality TV shows on one’s desire to undergo cosmetic surgery, Exercise 12.17 (p. 725). Recall that psychologists used multiple regression to model desire to have cosmetic surgery (y) as a function of gender(x1) , self-esteem(x2) , body satisfaction(x3) , and impression of reality TV (x4). The SPSS printout below shows a confidence interval for E(y) for each of the first five students in the study.

  1. Interpret the confidence interval for E(y) for student 1.
  2. Interpret the confidence interval for E(y) for student 4

Accuracy of software effort estimates. Refer to the Journal of Empirical Software Engineering (Vol. 9, 2004) study of the accuracy of new software effort estimates, Exercise 12.114 (p. 781). Recall that stepwise regression was used to develop a model for the relative error in estimating effort (y) as a function of company role of estimator (x1 = 1 if developer, 0 if project leader) and previous accuracy (x8 = 1 if more than 20% accurate, 0 if less than 20% accurate). The stepwise regression yielded the prediction equation y^= 0.12 - 0.28x1+ 0.27x8. The researcher is concerned that the sign of the estimated β multiplied by x1 is the opposite from what is expected. (The researcher expects a project leader to have a smaller relative error of estimation than a developer.) Give at least one reason why this phenomenon occurred.

The Minitab printout below was obtained from fitting the modely=β0+β1x1+β2x2+β3x1x2+εto n = 15 data points.

a) What is the prediction equation?

b) Give an estimate of the slope of the line relating y to x1 when x2 =10 .

c) Plot the prediction equation for the case when x2 =1 . Do this twice more on the same graph for the cases when x2 =3 and x2 =5 .

d) Explain what it means to say that x1and x2interact. Explain why your graph of part c suggests that x1and x2interact.

e) Specify the null and alternative hypotheses you would use to test whetherx1andx2interact.

f)Conduct the hypothesis test of part e using α=0.01.

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