Exam 20: Model Building

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In general, to represent a nominal independent variable that has n possible categories, we would create n dummy variables.

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The owner of an air conditioner business wants to investigate the relationship between the weekly number of air conditioners sold, temperature and the seasons of the year. A random sample of 14 weeks is taken, with the average temperature of that week (in degrees Celsius) and the quarter from which that week belonged, noted. There are three indicator variables, March, September and December. Excel is used to generate the following multiple linear regression output. SUMMARY OUTPUT Regression Statistics Multiple R 0.99 R Square 0.97 Adjusted RSquare 0.96 Standard Error 4.54 Observations 14.00 ANOVA df SS MS F Significance Regression 4.00 6999.27 1749.82 84.86 0.00 Residual 9.00 185.58 20.62 Total 13.00 7184.86 Coefficients Standard Error tStat P-value Lower 95\% Upper 95\% Intercept -17.94 8.54 -2.10 0.07 -37.27 1.38 Temperature 1.00 0.35 2.84 0.02 0.20 1.79 March 1.01 4.60 0.22 0.83 -9.39 11.40 September 7.22 5.58 1.29 0.23 -5.40 19.84 Deomber 27.87 6.55 4.26 0.00 13.06 42.68 Using the p-values from Excel, state which of the quarters are statistically significant at α of 5%.

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The model y=β0+β1x1+β2x2+β3x1x2y = \beta _ { 0 } + \beta _ { 1 } x _ { 1 } + \beta _ { 2 } x _ { 2 } + \beta _ { 3 } x _ { 1 } x _ { 2 } + ε\varepsilon is referred to as a:

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A regression analysis involving 40 observations and five independent variables revealed that the total variation in the dependent variable y is 1080 and that the mean square for error is 30. Create the ANOVA table.

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A traffic consultant has analysed the factors that affect the number of traffic fatalities. She has come to the conclusion that two important variables are the number of cars and the number of tractor-trailer trucks. She proposed the second-order model with interaction: y=y = β0+β1x1+β2x2+β3x12+β4x22+β5x1x2+ε\beta _ { 0 } + \beta _ { 1 } x _ { 1 } + \beta _ { 2 } x _ { 2 } + \beta _ { 3 } x _ { 1 } ^ { 2 } + \beta _ { 4 } x _ { 2 } ^ { 2 } + \beta _ { 5 } x _ { 1 } x _ { 2 } + \varepsilon . Where: y = number of annual fatalities per shire. x1x _ { 1 } = number of cars registered in the shire (in units of 10 000). x2x _ { 2 } = number of trucks registered in the shire (in units of 1000). The computer output (based on a random sample of 35 shires) is shown below. THE REGRESSION EQUATION IS y=y = 69.7+11.3x1+7.61x21.15x120.51x220.13x1x269.7 + 11.3 x _ { 1 } + 7.61 x _ { 2 } - 1.15 x _ { 1 } ^ { 2 } - 0.51 x _ { 2 } ^ { 2 } - 0.13 x _ { 1 } x _ { 2 } . Predictor Coef SiDev T Constant 69.7 41.3 1.688 11.3 5.1 2.216 7.61 2.55 2.984 -1.15 0.64 -1.797 -0.51 0.20 -2.55 -0.13 0.10 -1.30 S = 15.2 R-Sq = 47.2%. ANALYSIS OF VARIANCE Source of Variation df SS MS F Regression 5 5959 1191.800 5.181 Error 29 6671 230.034 Total 34 12630 Test at the 1% significance level to determine whether the interaction term should be retained in the model.

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An economist is in the process of developing a model to predict the price of gold. She believes that the two most important variables are the price of a barrel of oil (x1)\left( x _ { 1 } \right) and the interest rate (x2)\left( x _ { 2 } \right) She proposes the first-order model with interaction: y=β0+β1x1+β2x2+β3x1x3+εy = \beta _ { 0 } + \beta _ { 1 } x _ { 1 } + \beta _ { 2 } x _ { 2 } + \beta _ { 3 } x _ { 1 } x _ { 3 } + \varepsilon . A random sample of 20 daily observations was taken. The computer output is shown below. THE REGRESSION EQUATION IS y=y = 115.6+22.3x1+14.7x21.36x1x2115.6 + 22.3 x _ { 1 } + 14.7 x _ { 2 } - 1.36 x _ { 1 } x _ { 2 } . Predictor Coef SiDev T Constant 115.6 78.1 1.480 22.3 7.1 3.141 14.7 6.3 2.333 -1.36 0.52 -2.615 S = 20.9 R-Sq = 55.4%. ANALYSIS OF VARIANCE Source of Variation df SS MS F Regression 3 8661 2887.0 6.626 Error 16 6971 435.7 Total 19 15632 Interpret the coefficient b1b _ { 1 } .

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In explaining starting salaries for graduates of computer science programs, which of the following independent variables would not be adequately represented with a dummy variable?

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In the first-order model y^\hat{y} = 60 + 40x1 -10x2 + 5x1x2, a unit increase in x1, while holding x2 constant at 1, increases the value of yy on average by 45 units.

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A professor of accounting wanted to develop a multiple regression model to predict the students' grades in her fourth-year accounting course. She decides that the two most important factors are the student's grade point average (GPA) in the first three years and the student's major. She proposes the model: y=β0+β1x1+β2x2+β3x3+εy = \beta _ { 0 } + \beta _ { 1 } x _ { 1 } + \beta _ { 2 } x _ { 2 } + \beta _ { 3 } x _ { 3 } + \varepsilon . where y = fourth-year accounting course mark (out of 100). x1x _ { 1 } = GPA in first three years (range 0 to 12). x2x _ { 2 } = 1 if student's major is accounting. = 0 if not. x3x _ { 3 } = 1 if student's major is finance. = 0 if not. The computer output is shown below. THE REGRESSION EQUATION IS y=y = 9.14+6.73x1+10.42x2+5.16x39.14 + 6.73 x _ { 1 } + 10.42 x _ { 2 } + 5.16 x _ { 3 } . Predictor Coef SDev T Constant 9.14 7.10 1.287 6.73 1.91 3.524 10.42 4.16 2.505 5.16 3.93 1.313 S = 15.0 R-Sq = 44.2%. ANALYSIS OF VARIANCE Source of Variation df SS MS F Regression 3 17098 5699.333 25.386 Error 96 21553 224.510 Total 99 38651 Do these results allow us to conclude at the 1% significance level that grade point average in the first three years is linearly related to fourth-year accounting course mark?

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An economist is analysing the incomes of professionals (physicians, dentists and lawyers). He realises that an important factor is the number of years of experience. However, he wants to know if there are differences among the three professional groups. He takes a random sample of 125 professionals and estimates the multiple regression model: y=β0+β1x1+β2x2+β3x3+εy = \beta _ { 0 } + \beta _ { 1 } x _ { 1 } + \beta _ { 2 } x _ { 2 } + \beta _ { 3 } x _ { 3 } + \varepsilon . where y = annual income (in $1000). x1x _ { 1 } = years of experience. x2x _ { 2 } = 1 if physician. = 0 if not. x3x _ { 3 } = 1 if dentist. = 0 if not. The computer output is shown below. THE REGRESSION EQUATION IS y=y = 71.65+2.07x1+10.16x27.44x371.65 + 2.07 x _ { 1 } + 10.16 x _ { 2 } - 7.44 x _ { 3 } . Predictor Coef StDev T Constant 71.65 18.56 3.860 2.07 0.81 2.556 10.16 3.16 3.215 -7.44 2.85 -2.611 S = 42.6 R-Sq = 30.9%. ANALYSIS OF VARIANCE Source of Variation df SS MS F Regression 3 98008 32669.333 18.008 Error 121 219508 1814.116 Total 124 317516 Do these results allow us to conclude at the 1% significance level that the model is useful in predicting the income of professionals?

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The owner of an air conditioner business wants to investigate the relationship between the weekly number of air conditioners sold with average weekly temperature and the seasons of the year. A random sample of 14 weeks is taken, with the average temperature of that week (in degrees Celsius) and the quarter from which that week belonged, noted. There are three indicator variables: March, September and December. Excel is used to generate the following multiple linear regression output. SUMMARY OUTPUT Regression Statistics Multiple R 0.99 R Square 0.97 Adjusted RSquare 0.96 Standard Error 4.54 Observations 14.00 ANOVA df SS MS F Significance Regression 4.00 6999.27 1749.82 84.86 0.00 Residual 9.00 185.58 20.62 Total 13.00 7184.86 Coefficients Standard Error tStat P-value Lower 95\% Upper 95\% Intercept -17.94 8.54 -2.10 0.07 -37.27 1.38 Temperature 1.00 0.35 2.84 0.02 0.20 1.79 March 1.01 4.60 0.22 0.83 -9.39 11.40 September 7.22 5.58 1.29 0.23 -5.40 19.84 Deomber 27.87 6.55 4.26 0.00 13.06 42.68 Test the significance of the coefficient of Temperature, at the 5% level of α. Justify your choice of the alternative hypothesis.

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Suppose that the sample regression equation of a second-order model is  Suppose that the sample regression equation of a second-order model is   = 2.50 + 0.15 x + 0.45 x ^ { 2 }  . The value 4.60 is the: =2.50+0.15x+0.45x2= 2.50 + 0.15 x + 0.45 x ^ { 2 } . The value 4.60 is the:

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A first-order model was used in a regression analysis involving 25 observations to study the relationship between a dependent variable y and three independent variables, x1x _ { 1 } , x2x _ { 2 } and x3x _ { 3 } . The analysis showed that the mean squares for regression is 160 and the sum of squares for error is 1050. In addition, the following is a partial computer printout. Predictor Coef StDev Constant 25 4 18 6 -12 4.8 6 5 Is there sufficient evidence at the 5% significance level to indicate that x3x _ { 3 } is positively linearly related to y?

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The model y = β\beta 0 + β\beta 1x + ε\varepsilon is referred to as a simple linear regression model.

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A first-order model was used in a regression analysis involving 25 observations to study the relationship between a dependent variable y and three independent variables, x1x _ { 1 } , x2x _ { 2 } and x3x _ { 3 } . The analysis showed that the mean squares for regression is 160 and the sum of squares for error is 1050. In addition, the following is a partial computer printout. Predictor Coef StDev Constant 25 4 18 6 -12 4.8 6 5 Is there enough evidence at the 5% significance level to conclude that the model is useful in predicting the value of y?

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In explaining students' test scores, which of the following independent variables would not be adequately represented by an indicator variable?

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Consider the following data for two variables, x and y. x 7 10 3 5 3 10 4 14 5 8 y 35.0 28.5 45.0 45.0 55.0 25.0 37.5 27.5 30.0 27.5 Use Excel to find the coefficient of determination. What does this statistic tell you about this simple linear model?

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Suppose that the sample regression equation of a model is  Suppose that the sample regression equation of a model is   =4.7+2.2 x_{1}+2.6 x_{2}-x_{1} x_{2}  . If we examine the relationship between y and  x _ { 2 }  for  x _ { 1 }  = 1, 2 and 3, we observe that the three equations produced not only differ in the intercept term, but the coefficient of  x _ { 2 }  also varies. =4.7+2.2x1+2.6x2x1x2=4.7+2.2 x_{1}+2.6 x_{2}-x_{1} x_{2} . If we examine the relationship between y and x2x _ { 2 } for x1x _ { 1 } = 1, 2 and 3, we observe that the three equations produced not only differ in the intercept term, but the coefficient of x2x _ { 2 } also varies.

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Regression analysis allows the statistics practitioner to use mathematical models to realistically describe relationships between the dependent variable and independent variables.

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An economist is analysing the incomes of professionals (physicians, dentists and lawyers). He realises that an important factor is the number of years of experience. However, he wants to know if there are differences among the three professional groups. He takes a random sample of 125 professionals and estimates the multiple regression model: y=β0+β1x1+β2x2+β3x3+εy = \beta _ { 0 } + \beta _ { 1 } x _ { 1 } + \beta _ { 2 } x _ { 2 } + \beta _ { 3 } x _ { 3 } + \varepsilon . where y = annual income (in $1000). x1x _ { 1 } = years of experience. x2x _ { 2 } = 1 if physician. = 0 if not. x3x _ { 3 } = 1 if dentist. = 0 if not. The computer output is shown below. THE REGRESSION EQUATION IS y=y = 71.65+2.07x1+10.16x27.44x371.65 + 2.07 x _ { 1 } + 10.16 x _ { 2 } - 7.44 x _ { 3 } . Predictor Coef StDev T Constant 71.65 18.56 3.860 2.07 0.81 2.556 10.16 3.16 3.215 -7.44 2.85 -2.611 S = 42.6 R-Sq = 30.9%. ANALYSIS OF VARIANCE Source of Variation df SS MS F Regression 3 98008 32669.333 18.008 Error 121 219508 1814.116 Total 124 317516 Is there enough evidence at the1% significant level to conclude that physicians earn more on average than lawyers?

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