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Bbs11 ppt ch13
1.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc. Chap 13-1 Chapter 13 Simple Linear Regression Basic Business Statistics 11th Edition
2.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-2 Learning Objectives In this chapter, you learn: How to use regression analysis to predict the value of a dependent variable based on an independent variable The meaning of the regression coefficients b0 and b1 How to evaluate the assumptions of regression analysis and know what to do if the assumptions are violated To make inferences about the slope and correlation coefficient To estimate mean values and predict individual values
3.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-3 Correlation vs. Regression A scatter plot can be used to show the relationship between two variables Correlation analysis is used to measure the strength of the association (linear relationship) between two variables Correlation is only concerned with strength of the relationship No causal effect is implied with correlation Scatter plots were first presented in Ch. 2 Correlation was first presented in Ch. 3
4.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-4 Introduction to Regression Analysis Regression analysis is used to: Predict the value of a dependent variable based on the value of at least one independent variable Explain the impact of changes in an independent variable on the dependent variable Dependent variable: the variable we wish to predict or explain Independent variable: the variable used to predict or explain the dependent variable
5.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-5 Simple Linear Regression Model Only one independent variable, X Relationship between X and Y is described by a linear function Changes in Y are assumed to be related to changes in X
6.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-6 Types of Relationships Y X Y X Y Y X X Linear relationships Curvilinear relationships
7.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-7 Types of Relationships Y X Y X Y Y X X Strong relationships Weak relationships (continued)
8.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-8 Types of Relationships Y X Y X No relationship (continued)
9.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-9 ii10i εXββY Linear component Simple Linear Regression Model Population Y intercept Population Slope Coefficient Random Error term Dependent Variable Independent Variable Random Error component
10.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-10 (continued) Random Error for this Xi value Y X Observed Value of Y for Xi Predicted Value of Y for Xi ii10i εXββY Xi Slope = β1 Intercept = β0 εi Simple Linear Regression Model
11.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-11 i10i XbbYˆ The simple linear regression equation provides an estimate of the population regression line Simple Linear Regression Equation (Prediction Line) Estimate of the regression intercept Estimate of the regression slope Estimated (or predicted) Y value for observation i Value of X for observation i
12.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-12 The Least Squares Method b0 and b1 are obtained by finding the values of that minimize the sum of the squared differences between Y and : 2 i10i 2 ii ))Xb(b(Ymin)Yˆ(Ymin Yˆ
13.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-13 Finding the Least Squares Equation The coefficients b0 and b1 , and other regression results in this chapter, will be found using Excel or Minitab Formulas are shown in the text for those who are interested
14.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-14 b0 is the estimated average value of Y when the value of X is zero b1 is the estimated change in the average value of Y as a result of a one-unit change in X Interpretation of the Slope and the Intercept
15.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-15 Simple Linear Regression Example A real estate agent wishes to examine the relationship between the selling price of a home and its size (measured in square feet) A random sample of 10 houses is selected Dependent variable (Y) = house price in $1000s Independent variable (X) = square feet
16.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-16 Simple Linear Regression Example: Data House Price in $1000s (Y) Square Feet (X) 245 1400 312 1600 279 1700 308 1875 199 1100 219 1550 405 2350 324 2450 319 1425 255 1700
17.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-17 0 50 100 150 200 250 300 350 400 450 0 500 1000 1500 2000 2500 3000 Square Feet HousePrice($1000s) Simple Linear Regression Example: Scatter Plot House price model: Scatter Plot
18.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-18 Simple Linear Regression Example: Using Excel
19.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-19 Simple Linear Regression Example: Excel Output Regression Statistics Multiple R 0.76211 R Square 0.58082 Adjusted R Square 0.52842 Standard Error 41.33032 Observations 10 ANOVA df SS MS F Significance F Regression 1 18934.9348 18934.9348 11.0848 0.01039 Residual 8 13665.5652 1708.1957 Total 9 32600.5000 Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386 Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580 The regression equation is: feet)(square0.1097798.24833pricehouse
20.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-20 Simple Linear Regression Example: Minitab Output The regression equation is Price = 98.2 + 0.110 Square Feet Predictor Coef SE Coef T P Constant 98.25 58.03 1.69 0.129 Square Feet 0.10977 0.03297 3.33 0.010 S = 41.3303 R-Sq = 58.1% R-Sq(adj) = 52.8% Analysis of Variance Source DF SS MS F P Regression 1 18935 18935 11.08 0.010 Residual Error 8 13666 1708 Total 9 32600 The regression equation is: house price = 98.24833 + 0.10977 (square feet)
21.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-21 0 50 100 150 200 250 300 350 400 450 0 500 1000 1500 2000 2500 3000 Square Feet HousePrice($1000s) Simple Linear Regression Example: Graphical Representation House price model: Scatter Plot and Prediction Line feet)(square0.1097798.24833pricehouse Slope = 0.10977 Intercept = 98.248
22.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-22 Simple Linear Regression Example: Interpretation of bo b0 is the estimated average value of Y when the value of X is zero (if X = 0 is in the range of observed X values) Because a house cannot have a square footage of 0, b0 has no practical application feet)(square0.1097798.24833pricehouse
23.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-23 Simple Linear Regression Example: Interpreting b1 b1 estimates the change in the average value of Y as a result of a one-unit increase in X Here, b1 = 0.10977 tells us that the mean value of a house increases by .10977($1000) = $109.77, on average, for each additional one square foot of size feet)(square0.1097798.24833pricehouse
24.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-24 317.85 0)0.1098(20098.25 (sq.ft.)0.109898.25pricehouse Predict the price for a house with 2000 square feet: The predicted price for a house with 2000 square feet is 317.85($1,000s) = $317,850 Simple Linear Regression Example: Making Predictions
25.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-25 0 50 100 150 200 250 300 350 400 450 0 500 1000 1500 2000 2500 3000 Square Feet HousePrice($1000s) Simple Linear Regression Example: Making Predictions When using a regression model for prediction, only predict within the relevant range of data Relevant range for interpolation Do not try to extrapolate beyond the range of observed X’s
26.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-26 Measures of Variation Total variation is made up of two parts: SSESSRSST Total Sum of Squares Regression Sum of Squares Error Sum of Squares 2 i )YY(SST 2 ii )YˆY(SSE 2 i )YYˆ(SSR where: = Mean value of the dependent variable Yi = Observed value of the dependent variable = Predicted value of Y for the given Xi valueiYˆ Y
27.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-27 SST = total sum of squares (Total Variation) Measures the variation of the Yi values around their mean Y SSR = regression sum of squares (Explained Variation) Variation attributable to the relationship between X and Y SSE = error sum of squares (Unexplained Variation) Variation in Y attributable to factors other than X (continued) Measures of Variation
28.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-28 (continued) Xi Y X Yi SST = (Yi - Y)2 SSE = (Yi - Yi )2 SSR = (Yi - Y)2 _ _ _ Y Y Y _ Y Measures of Variation
29.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-29 The coefficient of determination is the portion of the total variation in the dependent variable that is explained by variation in the independent variable The coefficient of determination is also called r-squared and is denoted as r2 Coefficient of Determination, r2 1r0 2 note: squaresofsum squaresofregression2 total sum SST SSR r
30.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-30 r2 = 1 Examples of Approximate r2 Values Y X Y X r2 = 1 r2 = 1 Perfect linear relationship between X and Y: 100% of the variation in Y is explained by variation in X
31.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-31 Examples of Approximate r2 Values Y X Y X 0 < r2 < 1 Weaker linear relationships between X and Y: Some but not all of the variation in Y is explained by variation in X
32.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-32 Examples of Approximate r2 Values r2 = 0 No linear relationship between X and Y: The value of Y does not depend on X. (None of the variation in Y is explained by variation in X) Y X r2 = 0
33.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-33 Simple Linear Regression Example: Coefficient of Determination, r2 in Excel Regression Statistics Multiple R 0.76211 R Square 0.58082 Adjusted R Square 0.52842 Standard Error 41.33032 Observations 10 ANOVA df SS MS F Significance F Regression 1 18934.9348 18934.9348 11.0848 0.01039 Residual 8 13665.5652 1708.1957 Total 9 32600.5000 Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386 Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580 58.08% of the variation in house prices is explained by variation in square feet 0.58082 32600.5000 18934.9348 SST SSR r2
34.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-34 Simple Linear Regression Example: Coefficient of Determination, r2 in Minitab The regression equation is Price = 98.2 + 0.110 Square Feet Predictor Coef SE Coef T P Constant 98.25 58.03 1.69 0.129 Square Feet 0.10977 0.03297 3.33 0.010 S = 41.3303 R-Sq = 58.1% R-Sq(adj) = 52.8% Analysis of Variance Source DF SS MS F P Regression 1 18935 18935 11.08 0.010 Residual Error 8 13666 1708 Total 9 32600 0.58082 32600.5000 18934.9348 SST SSR r2 58.08% of the variation in house prices is explained by variation in square feet
35.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-35 Standard Error of Estimate The standard deviation of the variation of observations around the regression line is estimated by 2 )ˆ( 2 1 2 n YY n SSE S n i ii YX Where SSE = error sum of squares n = sample size
36.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-36 Simple Linear Regression Example: Standard Error of Estimate in Excel Regression Statistics Multiple R 0.76211 R Square 0.58082 Adjusted R Square 0.52842 Standard Error 41.33032 Observations 10 ANOVA df SS MS F Significance F Regression 1 18934.9348 18934.9348 11.0848 0.01039 Residual 8 13665.5652 1708.1957 Total 9 32600.5000 Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386 Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580 41.33032SYX
37.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-37 Simple Linear Regression Example: Standard Error of Estimate in Minitab The regression equation is Price = 98.2 + 0.110 Square Feet Predictor Coef SE Coef T P Constant 98.25 58.03 1.69 0.129 Square Feet 0.10977 0.03297 3.33 0.010 S = 41.3303 R-Sq = 58.1% R-Sq(adj) = 52.8% Analysis of Variance Source DF SS MS F P Regression 1 18935 18935 11.08 0.010 Residual Error 8 13666 1708 Total 9 32600 41.33032SYX
38.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-38 Comparing Standard Errors YY X X YX Ssmall YX Slarge SYX is a measure of the variation of observed Y values from the regression line The magnitude of SYX should always be judged relative to the size of the Y values in the sample data i.e., SYX = $41.33K is moderately small relative to house prices in the $200K - $400K range
39.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-39 Assumptions of Regression L.I.N.E Linearity The relationship between X and Y is linear Independence of Errors Error values are statistically independent Normality of Error Error values are normally distributed for any given value of X Equal Variance (also called homoscedasticity) The probability distribution of the errors has constant variance
40.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-40 Residual Analysis The residual for observation i, ei, is the difference between its observed and predicted value Check the assumptions of regression by examining the residuals Examine for linearity assumption Evaluate independence assumption Evaluate normal distribution assumption Examine for constant variance for all levels of X (homoscedasticity) Graphical Analysis of Residuals Can plot residuals vs. X iii YˆYe
41.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-41 Residual Analysis for Linearity Not Linear Linear x residuals x Y x Y x residuals
42.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-42 Residual Analysis for Independence Not Independent Independent X X residuals residuals X residuals
43.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-43 Checking for Normality Examine the Stem-and-Leaf Display of the Residuals Examine the Boxplot of the Residuals Examine the Histogram of the Residuals Construct a Normal Probability Plot of the Residuals
44.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-44 Residual Analysis for Normality Percent Residual When using a normal probability plot, normal errors will approximately display in a straight line -3 -2 -1 0 1 2 3 0 100
45.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-45 Residual Analysis for Equal Variance Non-constant variance Constant variance x x Y x x Y residuals residuals
46.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-46 House Price Model Residual Plot -60 -40 -20 0 20 40 60 80 0 1000 2000 3000 Square Feet Residuals Simple Linear Regression Example: Excel Residual Output RESIDUAL OUTPUT Predicted House Price Residuals 1 251.92316 -6.923162 2 273.87671 38.12329 3 284.85348 -5.853484 4 304.06284 3.937162 5 218.99284 -19.99284 6 268.38832 -49.38832 7 356.20251 48.79749 8 367.17929 -43.17929 9 254.6674 64.33264 10 284.85348 -29.85348 Does not appear to violate any regression assumptions
47.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-47 Used when data are collected over time to detect if autocorrelation is present Autocorrelation exists if residuals in one time period are related to residuals in another period Measuring Autocorrelation: The Durbin-Watson Statistic
48.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-48 Autocorrelation Autocorrelation is correlation of the errors (residuals) over time Violates the regression assumption that residuals are random and independent Time (t) Residual Plot -15 -10 -5 0 5 10 15 0 2 4 6 8 Time (t) Residuals Here, residuals show a cyclic pattern, not random. Cyclical patterns are a sign of positive autocorrelation
49.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-49 The Durbin-Watson Statistic n 1i 2 i n 2i 2 1ii e )ee( D The possible range is 0 ≤ D ≤ 4 D should be close to 2 if H0 is true D less than 2 may signal positive autocorrelation, D greater than 2 may signal negative autocorrelation The Durbin-Watson statistic is used to test for autocorrelation H0: residuals are not correlated H1: positive autocorrelation is present
50.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-50 Testing for Positive Autocorrelation Calculate the Durbin-Watson test statistic = D (The Durbin-Watson Statistic can be found using Excel or Minitab) Decision rule: reject H0 if D < dL H0: positive autocorrelation does not exist H1: positive autocorrelation is present 0 dU 2dL Reject H0 Do not reject H0 Find the values dL and dU from the Durbin-Watson table (for sample size n and number of independent variables k) Inconclusive
51.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-51 Suppose we have the following time series data: Is there autocorrelation? y = 30.65 + 4.7038x R 2 = 0.8976 0 20 40 60 80 100 120 140 160 0 5 10 15 20 25 30 Time Sales Testing for Positive Autocorrelation (continued)
52.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-52 Example with n = 25: Durbin-Watson Calculations Sum of Squared Difference of Residuals 3296.18 Sum of Squared Residuals 3279.98 Durbin-Watson Statistic 1.00494 y = 30.65 + 4.7038x R 2 = 0.8976 0 20 40 60 80 100 120 140 160 0 5 10 15 20 25 30 Time Sales Testing for Positive Autocorrelation (continued) Excel/PHStat output: 1.00494 3279.98 3296.18 e )e(e D n 1i 2 i n 2i 2 1ii
53.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-53 Here, n = 25 and there is k = 1 one independent variable Using the Durbin-Watson table, dL = 1.29 and dU = 1.45 D = 1.00494 < dL = 1.29, so reject H0 and conclude that significant positive autocorrelation exists Testing for Positive Autocorrelation (continued) Decision: reject H0 since D = 1.00494 < dL 0 dU=1.45 2dL=1.29 Reject H0 Do not reject H0Inconclusive
54.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-54 Inferences About the Slope The standard error of the regression slope coefficient (b1) is estimated by 2 i YXYX b )X(X S SSX S S 1 where: = Estimate of the standard error of the slope = Standard error of the estimate 1bS 2n SSE SYX
55.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-55 Inferences About the Slope: t Test t test for a population slope Is there a linear relationship between X and Y? Null and alternative hypotheses H0: β1 = 0 (no linear relationship) H1: β1 ≠ 0 (linear relationship does exist) Test statistic 1 b 11 STAT S βb t 2nd.f. where: b1 = regression slope coefficient β1 = hypothesized slope Sb1 = standard error of the slope
56.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-56 Inferences About the Slope: t Test Example House Price in $1000s (y) Square Feet (x) 245 1400 312 1600 279 1700 308 1875 199 1100 219 1550 405 2350 324 2450 319 1425 255 1700 (sq.ft.)0.109898.25pricehouse Estimated Regression Equation: The slope of this model is 0.1098 Is there a relationship between the square footage of the house and its sales price?
57.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-57 Inferences About the Slope: t Test Example H0: β1 = 0 H1: β1 ≠ 0From Excel output: Coefficients Standard Error t Stat P-value Intercept 98.24833 58.03348 1.69296 0.12892 Square Feet 0.10977 0.03297 3.32938 0.01039 1bSb1 329383 032970 0109770 S βb t 1 b 11 STAT . . . Predictor Coef SE Coef T P Constant 98.25 58.03 1.69 0.129 Square Feet 0.10977 0.03297 3.33 0.010 From Minitab output: b1 1bS
58.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-58 Inferences About the Slope: t Test Example Test Statistic: tSTAT = 3.329 There is sufficient evidence that square footage affects house price Decision: Reject H0 Reject H0Reject H0 a/2=.025 -tα/2 Do not reject H0 0 tα/2 a/2=.025 -2.3060 2.3060 3.329 d.f. = 10- 2 = 8 H0: β1 = 0 H1: β1 ≠ 0
59.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-59 Inferences About the Slope: t Test ExampleH0: β1 = 0 H1: β1 ≠ 0 From Excel output: Coefficients Standard Error t Stat P-value Intercept 98.24833 58.03348 1.69296 0.12892 Square Feet 0.10977 0.03297 3.32938 0.01039 p-value There is sufficient evidence that square footage affects house price. Decision: Reject H0, since p-value < α Predictor Coef SE Coef T P Constant 98.25 58.03 1.69 0.129 Square Feet 0.10977 0.03297 3.33 0.010 From Minitab output:
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Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-60 F Test for Significance F Test statistic: where MSE MSR FSTAT 1kn SSE MSE k SSR MSR where FSTAT follows an F distribution with k numerator and (n – k - 1) denominator degrees of freedom (k = the number of independent variables in the regression model)
61.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-61 F-Test for Significance Excel Output Regression Statistics Multiple R 0.76211 R Square 0.58082 Adjusted R Square 0.52842 Standard Error 41.33032 Observations 10 ANOVA df SS MS F Significance F Regression 1 18934.9348 18934.9348 11.0848 0.01039 Residual 8 13665.5652 1708.1957 Total 9 32600.5000 11.0848 1708.1957 18934.9348 MSE MSR FSTAT With 1 and 8 degrees of freedom p-value for the F-Test
62.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-62 F-Test for Significance Minitab Output Analysis of Variance Source DF SS MS F P Regression 1 18935 18935 11.08 0.010 Residual Error 8 13666 1708 Total 9 32600 11.0848 1708.1957 18934.9348 MSE MSR FSTAT With 1 and 8 degrees of freedom p-value for the F-Test
63.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-63 H0: β1 = 0 H1: β1 ≠ 0 a = .05 df1= 1 df2 = 8 Test Statistic: Decision: Conclusion: Reject H0 at a = 0.05 There is sufficient evidence that house size affects selling price0 a = .05 F.05 = 5.32 Reject H0Do not reject H0 11.08FSTAT MSE MSR Critical Value: Fa = 5.32 F Test for Significance (continued) F
64.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-64 Confidence Interval Estimate for the Slope Confidence Interval Estimate of the Slope: Excel Printout for House Prices: At 95% level of confidence, the confidence interval for the slope is (0.0337, 0.1858) 1 b2/1 Sb αt Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386 Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580 d.f. = n - 2
65.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-65 Since the units of the house price variable is $1000s, we are 95% confident that the average impact on sales price is between $33.74 and $185.80 per square foot of house size Coefficients Standard Error t Stat P-value Lower 95% Upper 95% Intercept 98.24833 58.03348 1.69296 0.12892 -35.57720 232.07386 Square Feet 0.10977 0.03297 3.32938 0.01039 0.03374 0.18580 This 95% confidence interval does not include 0. Conclusion: There is a significant relationship between house price and square feet at the .05 level of significance Confidence Interval Estimate for the Slope (continued)
66.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-66 t Test for a Correlation Coefficient Hypotheses H0: ρ = 0 (no correlation between X and Y) H1: ρ ≠ 0 (correlation exists) Test statistic (with n – 2 degrees of freedom) 2n r1 ρ-r t 2 STAT 0bifrr 0bifrr where 1 2 1 2
67.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-67 t-test For A Correlation Coefficient Is there evidence of a linear relationship between square feet and house price at the .05 level of significance? H0: ρ = 0 (No correlation) H1: ρ ≠ 0 (correlation exists) a =.05 , df = 10 - 2 = 8 3.329 210 .7621 0.762 2n r1 ρr t 22 STAT (continued)
68.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-68 t-test For A Correlation Coefficient Conclusion: There is evidence of a linear association at the 5% level of significance Decision: Reject H0 Reject H0Reject H0 a/2=.025 -tα/2 Do not reject H0 0 tα/2 a/2=.025 -2.3060 2.3060 3.329 d.f. = 10-2 = 8 3.329 210 .7621 0.762 2n r1 ρr t 22 STAT (continued)
69.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-69 Estimating Mean Values and Predicting Individual Values Y XXi Y = b0+b1Xi Confidence Interval for the mean of Y, given Xi Prediction Interval for an individual Y, given Xi Goal: Form intervals around Y to express uncertainty about the value of Y for a given Xi Y
70.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-70 Confidence Interval for the Average Y, Given X Confidence interval estimate for the mean value of Y given a particular Xi Size of interval varies according to distance away from mean, X ihtY YX2/ XX|Y Sˆ :μforintervalConfidence i α 2 i 2 i 2 i i )X(X )X(X n 1 SSX )X(X n 1 h
71.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-71 Prediction Interval for an Individual Y, Given X Confidence interval estimate for an Individual value of Y given a particular Xi This extra term adds to the interval width to reflect the added uncertainty for an individual case ihtY 1Sˆ :YforintervalConfidence YX2/ XX i α
72.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-72 Estimation of Mean Values: Example Find the 95% confidence interval for the mean price of 2,000 square-foot houses Predicted Price Yi = 317.85 ($1,000s) Confidence Interval Estimate for μY|X=X 37.12317.85 )X(X )X(X n 1 StYˆ 2 i 2 i YX0.025 The confidence interval endpoints are 280.66 and 354.90, or from $280,660 to $354,900 i
73.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-73 Estimation of Individual Values: Example Find the 95% prediction interval for an individual house with 2,000 square feet Predicted Price Yi = 317.85 ($1,000s) Prediction Interval Estimate for YX=X 102.28317.85 )X(X )X(X n 1 1StYˆ 2 i 2 i YX0.025 The prediction interval endpoints are 215.50 and 420.07, or from $215,500 to $420,070 i
74.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-74 Finding Confidence and Prediction Intervals in Excel From Excel, use PHStat | regression | simple linear regression … Check the “confidence and prediction interval for X=” box and enter the X-value and confidence level desired
75.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-75 Input values Finding Confidence and Prediction Intervals in Excel (continued) Confidence Interval Estimate for μY|X=Xi Prediction Interval Estimate for YX=Xi Y
76.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-76 Finding Confidence and Prediction Intervals in Minitab Predicted Values for New Observations New Obs Fit SE Fit 95% CI 95% PI 1 317.8 16.1 (280.7, 354.9) (215.5, 420.1) Values of Predictors for New Observations New Square Obs Feet 1 2000 Y Input values Confidence Interval Estimate for μY|X=Xi Prediction Interval Estimate for YX=Xi
77.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-77 Pitfalls of Regression Analysis Lacking an awareness of the assumptions underlying least-squares regression Not knowing how to evaluate the assumptions Not knowing the alternatives to least-squares regression if a particular assumption is violated Using a regression model without knowledge of the subject matter Extrapolating outside the relevant range
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Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-78 Strategies for Avoiding the Pitfalls of Regression Start with a scatter plot of X vs. Y to observe possible relationship Perform residual analysis to check the assumptions Plot the residuals vs. X to check for violations of assumptions such as homoscedasticity Use a histogram, stem-and-leaf display, boxplot, or normal probability plot of the residuals to uncover possible non-normality
79.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-79 Strategies for Avoiding the Pitfalls of Regression If there is violation of any assumption, use alternative methods or models If there is no evidence of assumption violation, then test for the significance of the regression coefficients and construct confidence intervals and prediction intervals Avoid making predictions or forecasts outside the relevant range (continued)
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Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-80 Chapter Summary Introduced types of regression models Reviewed assumptions of regression and correlation Discussed determining the simple linear regression equation Described measures of variation Discussed residual analysis Addressed measuring autocorrelation
81.
Basic Business Statistics,
11e © 2009 Prentice-Hall, Inc.. Chap 13-81 Chapter Summary Described inference about the slope Discussed correlation -- measuring the strength of the association Addressed estimation of mean values and prediction of individual values Discussed possible pitfalls in regression and recommended strategies to avoid them (continued)