Statistics for Business and Economics 8 th Edition Chapter 11 Simple Regression Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch. 11-1

Chapter Goals After completing this chapter, you should be able to: Explain the simple linear regression model Obtain and interpret the simple linear regression equation for a set of data Describe R 2 as a measure of explanatory power of the regression model Understand the assumptions behind regression analysis Explain measures of variation and determine whether the independent variable is significant Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch. 11-2

Chapter Goals After completing this chapter, you should be able to: Calculate and interpret confidence intervals for the regression coefficients Use a regression equation for prediction Form forecast intervals around an estimated Y value for a given X Use graphical analysis to recognize potential problems in regression analysis Explain the correlation coefficient and perform a hypothesis test for zero population correlation (continued) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch. 11-3

Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Overview of Linear Models An equation can be fit to show the best linear relationship between two variables: Y = β 0 + β 1 X Where Y is the dependent variable and X is the independent variable β 0 is the Y-intercept β 1 is the slope 11.1 Ch. 11-4

Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Least Squares Regression Estimates for coefficients β 0 and β 1 are found using a Least Squares Regression technique The least-squares regression line, based on sample data, is Where b 1 is the slope of the line and b 0 is the y- intercept: Ch. 11-5

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 explain (also called the endogenous variable) Independent variable: the variable used to explain the dependent variable (also called the exogenous variable) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch. 11-6

Linear Regression Model The relationship between X and Y is described by a linear function Changes in Y are assumed to be influenced by changes in X Linear regression population equation model Where  0 and  1 are the population model coefficients and  is a random error term. Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Simple Linear Regression Model Linear component The population regression model: Population Y intercept Population Slope Coefficient Random Error term Dependent Variable Independent Variable Random Error component Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch. 11-8

Linear Regression Assumptions The true relationship form is linear (Y is a linear function of X, plus random error) The error terms, ε i are independent of the x values The error terms are random variables with mean 0 and constant variance, σ 2 (the uniform variance property is called homoscedasticity) The random error terms, ε i, are not correlated with one another, so that Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch. 11-9

Simple Linear Regression Model (continued) Random Error for this X i value Y X Observed Value of Y for x i Predicted Value of Y for x i xixi Slope = β 1 Intercept = β 0 εiεi Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Simple Linear Regression Equation The simple linear regression equation provides an estimate of the population regression 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 The individual random error terms e i have a mean of zero Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Least Squares Coefficient Estimators b 0 and b 1 are obtained by finding the values of b 0 and b 1 that minimize the sum of the squared residuals (errors), SSE: Differential calculus is used to obtain the coefficient estimators b 0 and b 1 that minimize SSE Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Least Squares Coefficient Estimators The slope coefficient estimator is And the constant or y-intercept is The regression line always goes through the mean x, y (continued) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Computer Computation of Regression Coefficients The coefficients b 0 and b 1, and other regression results in this chapter, will be found using a computer Hand calculations are tedious Statistical routines are built into Excel Other statistical analysis software can be used Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Interpretation of the Slope and the Intercept b 0 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) b 1 is the estimated change in the average value of y as a result of a one- unit change in x Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

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 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Sample Data for House Price Model House Price in $1000s (Y) Square Feet (X) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Sample Data for House Price Model House House Price in $1000s (Y) Mean Y 286,5 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Scatter Plot with no independent Variable Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Total Sum of Squares House House Price in $1000s (y) Mean of House Price (ybar) Residuals (y- ybar) Sum of Squares (y-ybar)^ ,5-41,51722, ,525,5650, ,5-7,556, ,521,5462, ,5-87,57656, ,5-67,54556, ,5118,514042, ,537,51406, ,532,51056, ,5-31,5992,3 Sum032600,5 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

A Better Model? If there is no independent variable, our best prediction is the mean of the dependent variable, that is the average housing price. We can do a lot better!! I mean our prediction can be much better. We can reduce the sum of squared errors. Let us introduce the independent variable now. Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Scatter Plot Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Calculating b1 (Y)(X) y -ybarx-xbar (y -ybar)(x- xbar)(x-xbar)^ , , , , , , , , , , , , , , , , , , Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Calculating Least Squares Coefficient Estimators The slope coefficient estimator is b1= / =0, And the constant or y-intercept is b0=286,5- 0, *1715= 98, Our linear regression equation is ycap=98, , x Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Graphical Presentation House price model: scatter plot Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Regression Squared Error House Square Footage(X) Observed Housing Price (y) Predicted Housing Price (ycap) Error (y-ycap) Squared Error (y-ycap)^ , ,926347, , , , , ,857334, , , , , ,995399, , , , , , , , , , , , , , ,857891,46015 SSE13665,565 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Coefficient of Determination SST=32600,5 SSE=13665,565 SSR=SST-SSE= 18934,9346 R^2= SSR/SST= 0, NOT BAD How well does the estimated regression fit the data? Idea: Splitting SST between SSE and SSR 58 % of total sum of squares can be explained by using the regression equation to predict the housing price. The remainder is the error. Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Regression Using Excel Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch Excel will be used to generate the coefficients and measures of goodness of fit for regression Data / Data Analysis / Regression

Regression Using Excel Data / Data Analysis / Regression Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch (continued) Provide desired input:

Excel Output Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Excel Output Regression Statistics Multiple R R Square Adjusted R Square Standard Error Observations10 ANOVA dfSSMSFSignificance F Regression Residual Total CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet The regression equation is: Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall (continued) Ch

Graphical Presentation House price model: scatter plot and regression line Slope = Intercept = Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Interpretation of the Intercept, b 0 b 0 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) Here, no houses had 0 square feet, so b 0 = just indicates that, for houses within the range of sizes observed, $98, is the portion of the house price not explained by square feet Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Interpretation of the Slope Coefficient, b 1 b 1 measures the estimated change in the average value of Y as a result of a one- unit change in X Here, b 1 = tells us that the average value of a house increases by.10977($1000) = $109.77, on average, for each additional one square foot of size Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Explanatory Power of a Linear Regression Equation Total variation is made up of two parts: Total Sum of Squares Regression Sum of Squares Error (residual) Sum of Squares where: = Average value of the dependent variable y i = Observed values of the dependent variable i = Predicted value of y for the given x i value Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Analysis of Variance SST = total sum of squares Measures the variation of the y i values around their mean, y SSR = regression sum of squares Explained variation attributable to the linear relationship between x and y SSE = error sum of squares Variation attributable to factors other than the linear relationship between x and y Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Analysis of Variance (continued) xixi y X yiyi SST =  (y i - y) 2 SSE =  (y i - y i ) 2  SSR =  (y i - y) 2  _ _ _ y  Y y _ y  Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch Explained variation Unexplained variation

Coefficient of Determination, R 2 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 R 2 note: Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Examples of Approximate r 2 Values r 2 = 1 Y X Y X Perfect linear relationship between X and Y: 100% of the variation in Y is explained by variation in X Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Examples of Approximate r 2 Values Y X Y X 0 < r 2 < 1 Weaker linear relationships between X and Y: Some but not all of the variation in Y is explained by variation in X Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Examples of Approximate r 2 Values r 2 = 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 r 2 = 0 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Excel Output Regression Statistics Multiple R R Square Adjusted R Square Standard Error Observations10 ANOVA dfSSMSFSignificance F Regression Residual Total CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet % of the variation in house prices is explained by variation in square feet Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Correlation and R 2 The coefficient of determination, R 2, for a simple regression is equal to the simple correlation squared Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Estimation of Model Error Variance An estimator for the variance of the population model error is Division by n – 2 instead of n – 1 is because the simple regression model uses two estimated parameters, b 0 and b 1, instead of one is called the standard error of the estimate Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Excel Output Regression Statistics Multiple R R Square Adjusted R Square Standard Error Observations10 ANOVA dfSSMSFSignificance F Regression Residual Total CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Comparing Standard Errors YY X X s e is a measure of the variation of observed y values from the regression line The magnitude of s e should always be judged relative to the size of the y values in the sample data i.e., s e = $41.33K is moderately small relative to house prices in the $200 - $300K range Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Statistical Inference: Hypothesis Tests and Confidence Intervals The variance of the regression slope coefficient (b 1 ) is estimated by where: = Estimate of the standard error of the least squares slope = Standard error of the estimate Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Statistical Inference: Hypothesis Tests and Confidence Intervals The variance of the regression slope coefficient (b 1 ) depends on two things: 1. The distance of the points from the regression line measured by s_e. Higher values imply greater variance 2. The total deviation of X values from the mean. The greater the spread in X values, the smaller the variance for the slope coefficient. It follows that smaller variance slope coefficient estimators imply a better regression model. Change in Y resulting from change in X is estimated by b_1. So, Its variance should be as small as possible. Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Excel Output Regression Statistics Multiple R R Square Adjusted R Square Standard Error Observations10 ANOVA dfSSMSFSignificance F Regression Residual Total CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Comparing Standard Errors of the Slope Y X Y X is a measure of the variation in the slope of regression lines from different possible samples Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Inference about the Slope: t Test t test for a population slope Is there a linear relationship between X and Y? Null and alternative hypotheses H 0 : β 1 = 0(no linear relationship) H 1 : β 1  0(linear relationship does exist) Test statistic where: b 1 = regression slope coefficient β 1 = hypothesized slope s b1 = standard error of the slope Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Inference about the Slope: t Test House Price in $1000s (y) Square Feet (x) Estimated Regression Equation: The slope of this model is Does square footage of the house significantly affect its sales price? (continued) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Inferences about the Slope: t Test Example H 0 : β 1 = 0 H 1 : β 1  0 From Excel output: CoefficientsStandard Errort StatP-value Intercept Square Feet t b1b1 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Inferences about the Slope: t Test Example H 0 : β 1 = 0 H 1 : β 1  0 Test Statistic: t = There is sufficient evidence that square footage affects house price From Excel output: Reject H 0 CoefficientsStandard Errort StatP-value Intercept Square Feet tb1b1 Decision: Conclusion: Reject H 0  /2=.025 -t n-2,α/2 Do not reject H 0 0  /2= d.f. = 10-2 = 8 t 8,.025 = (continued) t n-2,α/2 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Inferences about the Slope: t Test Example H 0 : β 1 = 0 H 1 : β 1  0 P-value = There is sufficient evidence that square footage affects house price From Excel output: Reject H 0 CoefficientsStandard Errort StatP-value Intercept Square Feet P-value Decision: P-value < α so Conclusion: (continued) This is a two-tail test, so the p-value is P(t > 3.329)+P(t < ) = (for 8 d.f.) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

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, ) CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet d.f. = n - 2 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Confidence Interval Estimate for the Slope Since the units of the house price variable is $1000s, we are 95% confident that the average impact on sales price is between $33.70 and $ per square foot of house size CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet 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 (continued) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Hypothesis Test for Population Slope Using the F Distribution F Test statistic: where where F 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) Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Hypothesis Test for Population Slope Using the F Distribution An alternate test for the hypothesis that the slope is zero: Use the F statistic The decision rule is reject H 0 if F ≥ F 1,n-2,α Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch (continued) H 0 : β 1 = 0 H 1 : β 1  0

Excel Output Regression Statistics Multiple R R Square Adjusted R Square Standard Error Observations10 ANOVA dfSSMSFSignificance F Regression Residual Total CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept Square Feet With 1 and 8 degrees of freedom P-value for the F-Test Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

F-Test for Significance H 0 : β 1 = 0 H 1 : β 1 ≠ 0  =.05 df 1 = 1 df 2 = 8 Test Statistic: Decision: Conclusion: Reject H 0 at  = 0.05 There is sufficient evidence that house size affects selling price 0  =.05 F.05 = 5.32 Reject H 0 Do not reject H 0 Critical Value: F 1,8,0.05 = 5.32 (continued) F Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Prediction The regression equation can be used to predict a value for y, given a particular x For a specified value, x n+1, the predicted value is Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Predictions Using Regression Analysis Predict the price for a house with 2000 square feet: The predicted price for a house with 2000 square feet is ($1,000s) = $317,850 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Relevant Data Range When using a regression model for prediction, only predict within the relevant range of data Relevant data range Risky to try to extrapolate far beyond the range of observed x values Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Estimating Mean Values and Predicting Individual Values Y X x i y = b 0 +b 1 x i  Confidence Interval for the expected value of y, given x i Prediction Interval for an single observed y, given x i Goal: Form intervals around y to express uncertainty about the value of y for a given x i y  Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Confidence Interval for the Average Y, Given X Confidence interval estimate for the expected value of y given a particular x i Notice that the formula involves the term so the size of interval varies according to the distance x n+1 is from the mean, x Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Prediction Interval for an Individual Y, Given X Confidence interval estimate for an actual observed value of y given a particular x i This extra term adds to the interval width to reflect the added uncertainty for an individual case Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Example: Confidence Interval for the Average Y, Given X Find the 95% confidence interval for the mean price of 2,000 square-foot houses Predicted Price y i = ($1,000s)  Confidence Interval Estimate for E(Y n+1 |X n+1 ) The confidence interval endpoints are and , or from $280,730 to $354,970 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Example: Prediction Interval for an Individual Y, Given X Find the 95% confidence interval for an individual house with 2,000 square feet Predicted Price y i = ($1,000s)  Confidence Interval Estimate for y n+1 The confidence interval endpoints are and , or from $215,570 to $420,130  Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Correlation Analysis Correlation analysis is used to measure 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 Correlation was first presented in Chapter 4 Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Correlation Analysis The population correlation coefficient is denoted ρ (the Greek letter rho) The sample correlation coefficient is where Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Test for Zero Population Correlation To test the null hypothesis of no linear association, the test statistic follows the Student’s t distribution with (n – 2 ) degrees of freedom: Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Decision Rules Lower-tail test: H 0 : ρ  0 H 1 : ρ < 0 Upper-tail test: H 0 : ρ ≤ 0 H 1 : ρ > 0 Two-tail test: H 0 : ρ = 0 H 1 : ρ ≠ 0 Hypothesis Test for Correlation  /2  -t  -t  /2 tt t  /2 Reject H 0 if t < -t n-2,  Reject H 0 if t > t n-2,  Reject H 0 if t < -t n-2,    or t > t n-2,  Where has n - 2 d.f. Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Beta Measure of Financial Risk A Beta Coefficient is a measure of how the returns of a particular firm respond to the returns of a broad stock index (such as the S&P 500) For a specific firm, the Beta Coefficient is the slope coefficient from a regression of the firm’s returns compared to the overall market returns over some specified time period Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Beta Coefficient Example Slope coefficient is the Beta Coefficient Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch Information about the quality of the regression model that provides the estimate of beta

Graphical Analysis The linear regression model is based on minimizing the sum of squared errors If outliers exist, their potentially large squared errors may have a strong influence on the fitted regression line Be sure to examine your data graphically for outliers and extreme points Decide, based on your model and logic, whether the extreme points should remain or be removed Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Chapter Summary Introduced the linear regression model Reviewed correlation and the assumptions of linear regression Discussed estimating the simple linear regression coefficients Described measures of variation Described inference about the slope Addressed estimation of mean values and prediction of individual values Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch

Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall Ch All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Printed in the United States of America.