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  <title>9.3 Internal and External Validity when the Regression is Used for Forecasting | Introduction to Econometrics with R</title>
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<li class="chapter" data-level="" data-path="index.html"><a href="index.html"><i class="fa fa-check"></i>Preface</a></li>
<li class="chapter" data-level="1" data-path="1-introduction.html"><a href="1-introduction.html"><i class="fa fa-check"></i><b>1</b> Introduction</a>
<ul>
<li class="chapter" data-level="1.1" data-path="1-1-colophon.html"><a href="1-1-colophon.html"><i class="fa fa-check"></i><b>1.1</b> Colophon</a></li>
<li class="chapter" data-level="1.2" data-path="1-2-a-very-short-introduction-to-r-and-rstudio.html"><a href="1-2-a-very-short-introduction-to-r-and-rstudio.html"><i class="fa fa-check"></i><b>1.2</b> A Very Short Introduction to <tt>R</tt> and <em>RStudio</em></a></li>
</ul></li>
<li class="chapter" data-level="2" data-path="2-pt.html"><a href="2-pt.html"><i class="fa fa-check"></i><b>2</b> Probability Theory</a>
<ul>
<li class="chapter" data-level="2.1" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html"><i class="fa fa-check"></i><b>2.1</b> Random Variables and Probability Distributions</a>
<ul>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#probability-distributions-of-discrete-random-variables"><i class="fa fa-check"></i>Probability Distributions of Discrete Random Variables</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#bernoulli-trials"><i class="fa fa-check"></i>Bernoulli Trials</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#expected-value-mean-and-variance"><i class="fa fa-check"></i>Expected Value, Mean and Variance</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#probability-distributions-of-continuous-random-variables"><i class="fa fa-check"></i>Probability Distributions of Continuous Random Variables</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#the-normal-distribution"><i class="fa fa-check"></i>The Normal Distribution</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#the-chi-squared-distribution"><i class="fa fa-check"></i>The Chi-Squared Distribution</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#thetdist"><i class="fa fa-check"></i>The Student t Distribution</a></li>
<li class="chapter" data-level="" data-path="2-1-random-variables-and-probability-distributions.html"><a href="2-1-random-variables-and-probability-distributions.html#the-f-distribution"><i class="fa fa-check"></i>The F Distribution</a></li>
</ul></li>
<li class="chapter" data-level="2.2" data-path="2-2-RSATDOSA.html"><a href="2-2-RSATDOSA.html"><i class="fa fa-check"></i><b>2.2</b> Random Sampling and the Distribution of Sample Averages</a>
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<li class="chapter" data-level="" data-path="2-2-RSATDOSA.html"><a href="2-2-RSATDOSA.html#mean-and-variance-of-the-sample-mean"><i class="fa fa-check"></i>Mean and Variance of the Sample Mean</a></li>
<li class="chapter" data-level="" data-path="2-2-RSATDOSA.html"><a href="2-2-RSATDOSA.html#large-sample-approximations-to-sampling-distributions"><i class="fa fa-check"></i>Large Sample Approximations to Sampling Distributions</a></li>
</ul></li>
<li class="chapter" data-level="2.3" data-path="2-3-exercises-2.html"><a href="2-3-exercises-2.html"><i class="fa fa-check"></i><b>2.3</b> Exercises</a></li>
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<li class="chapter" data-level="3" data-path="3-arosur.html"><a href="3-arosur.html"><i class="fa fa-check"></i><b>3</b> A Review of Statistics using R</a>
<ul>
<li class="chapter" data-level="3.1" data-path="3-1-estimation-of-the-population-mean.html"><a href="3-1-estimation-of-the-population-mean.html"><i class="fa fa-check"></i><b>3.1</b> Estimation of the Population Mean</a></li>
<li class="chapter" data-level="3.2" data-path="3-2-potsm.html"><a href="3-2-potsm.html"><i class="fa fa-check"></i><b>3.2</b> Properties of the Sample Mean</a></li>
<li class="chapter" data-level="3.3" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html"><i class="fa fa-check"></i><b>3.3</b> Hypothesis Tests Concerning the Population Mean</a>
<ul>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#the-p-value"><i class="fa fa-check"></i>The p-Value</a></li>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#calculating-the-p-value-when-the-standard-deviation-is-known"><i class="fa fa-check"></i>Calculating the p-Value when the Standard Deviation is Known</a></li>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#SVSSDASE"><i class="fa fa-check"></i>Sample Variance, Sample Standard Deviation and Standard Error</a></li>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#calculating-the-p-value-when-the-standard-deviation-is-unknown"><i class="fa fa-check"></i>Calculating the p-value When the Standard Deviation is Unknown</a></li>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#the-t-statistic"><i class="fa fa-check"></i>The t-statistic</a></li>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#hypothesis-testing-with-a-prespecified-significance-level"><i class="fa fa-check"></i>Hypothesis Testing with a Prespecified Significance Level</a></li>
<li class="chapter" data-level="" data-path="3-3-hypothesis-tests-concerning-the-population-mean.html"><a href="3-3-hypothesis-tests-concerning-the-population-mean.html#one-sided-alternatives"><i class="fa fa-check"></i>One-sided Alternatives</a></li>
</ul></li>
<li class="chapter" data-level="3.4" data-path="3-4-confidence-intervals-for-the-population-mean.html"><a href="3-4-confidence-intervals-for-the-population-mean.html"><i class="fa fa-check"></i><b>3.4</b> Confidence Intervals for the Population Mean</a></li>
<li class="chapter" data-level="3.5" data-path="3-5-cmfdp.html"><a href="3-5-cmfdp.html"><i class="fa fa-check"></i><b>3.5</b> Comparing Means from Different Populations</a></li>
<li class="chapter" data-level="3.6" data-path="3-6-aattggoe.html"><a href="3-6-aattggoe.html"><i class="fa fa-check"></i><b>3.6</b> An Application to the Gender Gap of Earnings</a></li>
<li class="chapter" data-level="3.7" data-path="3-7-scatterplots-sample-covariance-and-sample-correlation.html"><a href="3-7-scatterplots-sample-covariance-and-sample-correlation.html"><i class="fa fa-check"></i><b>3.7</b> Scatterplots, Sample Covariance and Sample Correlation</a></li>
<li class="chapter" data-level="3.8" data-path="3-8-exercises-3.html"><a href="3-8-exercises-3.html"><i class="fa fa-check"></i><b>3.8</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="4" data-path="4-lrwor.html"><a href="4-lrwor.html"><i class="fa fa-check"></i><b>4</b> Linear Regression with One Regressor</a>
<ul>
<li class="chapter" data-level="4.1" data-path="4-1-simple-linear-regression.html"><a href="4-1-simple-linear-regression.html"><i class="fa fa-check"></i><b>4.1</b> Simple Linear Regression</a></li>
<li class="chapter" data-level="4.2" data-path="4-2-estimating-the-coefficients-of-the-linear-regression-model.html"><a href="4-2-estimating-the-coefficients-of-the-linear-regression-model.html"><i class="fa fa-check"></i><b>4.2</b> Estimating the Coefficients of the Linear Regression Model</a>
<ul>
<li class="chapter" data-level="" data-path="4-2-estimating-the-coefficients-of-the-linear-regression-model.html"><a href="4-2-estimating-the-coefficients-of-the-linear-regression-model.html#the-ordinary-least-squares-estimator"><i class="fa fa-check"></i>The Ordinary Least Squares Estimator</a></li>
</ul></li>
<li class="chapter" data-level="4.3" data-path="4-3-measures-of-fit.html"><a href="4-3-measures-of-fit.html"><i class="fa fa-check"></i><b>4.3</b> Measures of Fit</a>
<ul>
<li class="chapter" data-level="" data-path="4-3-measures-of-fit.html"><a href="4-3-measures-of-fit.html#the-coefficient-of-determination"><i class="fa fa-check"></i>The Coefficient of Determination</a></li>
<li class="chapter" data-level="" data-path="4-3-measures-of-fit.html"><a href="4-3-measures-of-fit.html#the-standard-error-of-the-regression"><i class="fa fa-check"></i>The Standard Error of the Regression</a></li>
<li class="chapter" data-level="" data-path="4-3-measures-of-fit.html"><a href="4-3-measures-of-fit.html#application-to-the-test-score-data"><i class="fa fa-check"></i>Application to the Test Score Data</a></li>
</ul></li>
<li class="chapter" data-level="4.4" data-path="4-4-tlsa.html"><a href="4-4-tlsa.html"><i class="fa fa-check"></i><b>4.4</b> The Least Squares Assumptions</a>
<ul>
<li class="chapter" data-level="" data-path="4-4-tlsa.html"><a href="4-4-tlsa.html#assumption-1-the-error-term-has-conditional-mean-of-zero"><i class="fa fa-check"></i>Assumption 1: The Error Term has Conditional Mean of Zero</a></li>
<li class="chapter" data-level="" data-path="4-4-tlsa.html"><a href="4-4-tlsa.html#assumption-2-independently-and-identically-distributed-data"><i class="fa fa-check"></i>Assumption 2: Independently and Identically Distributed Data</a></li>
<li class="chapter" data-level="" data-path="4-4-tlsa.html"><a href="4-4-tlsa.html#assumption-3-large-outliers-are-unlikely"><i class="fa fa-check"></i>Assumption 3: Large Outliers are Unlikely</a></li>
</ul></li>
<li class="chapter" data-level="4.5" data-path="4-5-tsdotoe.html"><a href="4-5-tsdotoe.html"><i class="fa fa-check"></i><b>4.5</b> The Sampling Distribution of the OLS Estimator</a>
<ul>
<li class="chapter" data-level="" data-path="4-5-tsdotoe.html"><a href="4-5-tsdotoe.html#simulation-study-1"><i class="fa fa-check"></i>Simulation Study 1</a></li>
<li class="chapter" data-level="" data-path="4-5-tsdotoe.html"><a href="4-5-tsdotoe.html#simulation-study-2"><i class="fa fa-check"></i>Simulation Study 2</a></li>
<li class="chapter" data-level="" data-path="4-5-tsdotoe.html"><a href="4-5-tsdotoe.html#simulation-study-3"><i class="fa fa-check"></i>Simulation Study 3</a></li>
</ul></li>
<li class="chapter" data-level="4.6" data-path="4-6-exercises-4.html"><a href="4-6-exercises-4.html"><i class="fa fa-check"></i><b>4.6</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="5" data-path="5-htaciitslrm.html"><a href="5-htaciitslrm.html"><i class="fa fa-check"></i><b>5</b> Hypothesis Tests and Confidence Intervals in the Simple Linear Regression Model</a>
<ul>
<li class="chapter" data-level="5.1" data-path="5-1-testing-two-sided-hypotheses-concerning-the-slope-coefficient.html"><a href="5-1-testing-two-sided-hypotheses-concerning-the-slope-coefficient.html"><i class="fa fa-check"></i><b>5.1</b> Testing Two-Sided Hypotheses Concerning the Slope Coefficient</a></li>
<li class="chapter" data-level="5.2" data-path="5-2-cifrc.html"><a href="5-2-cifrc.html"><i class="fa fa-check"></i><b>5.2</b> Confidence Intervals for Regression Coefficients</a>
<ul>
<li class="chapter" data-level="" data-path="5-2-cifrc.html"><a href="5-2-cifrc.html#simulation-study-confidence-intervals"><i class="fa fa-check"></i>Simulation Study: Confidence Intervals</a></li>
</ul></li>
<li class="chapter" data-level="5.3" data-path="5-3-rwxiabv.html"><a href="5-3-rwxiabv.html"><i class="fa fa-check"></i><b>5.3</b> Regression when X is a Binary Variable</a></li>
<li class="chapter" data-level="5.4" data-path="5-4-hah.html"><a href="5-4-hah.html"><i class="fa fa-check"></i><b>5.4</b> Heteroskedasticity and Homoskedasticity</a>
<ul>
<li class="chapter" data-level="" data-path="5-4-hah.html"><a href="5-4-hah.html#a-real-world-example-for-heteroskedasticity"><i class="fa fa-check"></i>A Real-World Example for Heteroskedasticity</a></li>
<li class="chapter" data-level="" data-path="5-4-hah.html"><a href="5-4-hah.html#should-we-care-about-heteroskedasticity"><i class="fa fa-check"></i>Should We Care About Heteroskedasticity?</a></li>
<li class="chapter" data-level="" data-path="5-4-hah.html"><a href="5-4-hah.html#computation-of-heteroskedasticity-robust-standard-errors"><i class="fa fa-check"></i>Computation of Heteroskedasticity-Robust Standard Errors</a></li>
</ul></li>
<li class="chapter" data-level="5.5" data-path="5-5-the-gauss-markov-theorem.html"><a href="5-5-the-gauss-markov-theorem.html"><i class="fa fa-check"></i><b>5.5</b> The Gauss-Markov Theorem</a>
<ul>
<li class="chapter" data-level="" data-path="5-5-the-gauss-markov-theorem.html"><a href="5-5-the-gauss-markov-theorem.html#simulation-study-blue-estimator"><i class="fa fa-check"></i>Simulation Study: BLUE Estimator</a></li>
</ul></li>
<li class="chapter" data-level="5.6" data-path="5-6-using-the-t-statistic-in-regression-when-the-sample-size-is-small.html"><a href="5-6-using-the-t-statistic-in-regression-when-the-sample-size-is-small.html"><i class="fa fa-check"></i><b>5.6</b> Using the t-Statistic in Regression When the Sample Size Is Small</a></li>
<li class="chapter" data-level="5.7" data-path="5-7-exercises-5.html"><a href="5-7-exercises-5.html"><i class="fa fa-check"></i><b>5.7</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="6" data-path="6-rmwmr.html"><a href="6-rmwmr.html"><i class="fa fa-check"></i><b>6</b> Regression Models with Multiple Regressors</a>
<ul>
<li class="chapter" data-level="6.1" data-path="6-1-omitted-variable-bias.html"><a href="6-1-omitted-variable-bias.html"><i class="fa fa-check"></i><b>6.1</b> Omitted Variable Bias</a></li>
<li class="chapter" data-level="6.2" data-path="6-2-tmrm.html"><a href="6-2-tmrm.html"><i class="fa fa-check"></i><b>6.2</b> The Multiple Regression Model</a></li>
<li class="chapter" data-level="6.3" data-path="6-3-mofimr.html"><a href="6-3-mofimr.html"><i class="fa fa-check"></i><b>6.3</b> Measures of Fit in Multiple Regression</a></li>
<li class="chapter" data-level="6.4" data-path="6-4-ols-assumptions-in-multiple-regression.html"><a href="6-4-ols-assumptions-in-multiple-regression.html"><i class="fa fa-check"></i><b>6.4</b> OLS Assumptions in Multiple Regression</a>
<ul>
<li class="chapter" data-level="" data-path="6-4-ols-assumptions-in-multiple-regression.html"><a href="6-4-ols-assumptions-in-multiple-regression.html#multicollinearity"><i class="fa fa-check"></i>Multicollinearity</a></li>
<li class="chapter" data-level="" data-path="6-4-ols-assumptions-in-multiple-regression.html"><a href="6-4-ols-assumptions-in-multiple-regression.html#simulation-study-imperfect-multicollinearity"><i class="fa fa-check"></i>Simulation Study: Imperfect Multicollinearity</a></li>
</ul></li>
<li class="chapter" data-level="6.5" data-path="6-5-the-distribution-of-the-ols-estimators-in-multiple-regression.html"><a href="6-5-the-distribution-of-the-ols-estimators-in-multiple-regression.html"><i class="fa fa-check"></i><b>6.5</b> The Distribution of the OLS Estimators in Multiple Regression</a></li>
<li class="chapter" data-level="6.6" data-path="6-6-exercises-6.html"><a href="6-6-exercises-6.html"><i class="fa fa-check"></i><b>6.6</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="7" data-path="7-htaciimr.html"><a href="7-htaciimr.html"><i class="fa fa-check"></i><b>7</b> Hypothesis Tests and Confidence Intervals in Multiple Regression</a>
<ul>
<li class="chapter" data-level="7.1" data-path="7-1-hypothesis-tests-and-confidence-intervals-for-a-single-coefficient.html"><a href="7-1-hypothesis-tests-and-confidence-intervals-for-a-single-coefficient.html"><i class="fa fa-check"></i><b>7.1</b> Hypothesis Tests and Confidence Intervals for a Single Coefficient</a></li>
<li class="chapter" data-level="7.2" data-path="7-2-an-application-to-test-scores-and-the-student-teacher-ratio.html"><a href="7-2-an-application-to-test-scores-and-the-student-teacher-ratio.html"><i class="fa fa-check"></i><b>7.2</b> An Application to Test Scores and the Student-Teacher Ratio</a>
<ul>
<li class="chapter" data-level="" data-path="7-2-an-application-to-test-scores-and-the-student-teacher-ratio.html"><a href="7-2-an-application-to-test-scores-and-the-student-teacher-ratio.html#another-augmentation-of-the-model"><i class="fa fa-check"></i>Another Augmentation of the Model</a></li>
</ul></li>
<li class="chapter" data-level="7.3" data-path="7-3-joint-hypothesis-testing-using-the-f-statistic.html"><a href="7-3-joint-hypothesis-testing-using-the-f-statistic.html"><i class="fa fa-check"></i><b>7.3</b> Joint Hypothesis Testing Using the F-Statistic</a></li>
<li class="chapter" data-level="7.4" data-path="7-4-confidence-sets-for-multiple-coefficients.html"><a href="7-4-confidence-sets-for-multiple-coefficients.html"><i class="fa fa-check"></i><b>7.4</b> Confidence Sets for Multiple Coefficients</a></li>
<li class="chapter" data-level="7.5" data-path="7-5-model-specification-for-multiple-regression.html"><a href="7-5-model-specification-for-multiple-regression.html"><i class="fa fa-check"></i><b>7.5</b> Model Specification for Multiple Regression</a>
<ul>
<li class="chapter" data-level="" data-path="7-5-model-specification-for-multiple-regression.html"><a href="7-5-model-specification-for-multiple-regression.html#model-specification-in-theory-and-in-practice"><i class="fa fa-check"></i>Model Specification in Theory and in Practice</a></li>
</ul></li>
<li class="chapter" data-level="7.6" data-path="7-6-analysis-of-the-test-score-data-set.html"><a href="7-6-analysis-of-the-test-score-data-set.html"><i class="fa fa-check"></i><b>7.6</b> Analysis of the Test Score Data Set</a></li>
<li class="chapter" data-level="7.7" data-path="7-7-exercises-7.html"><a href="7-7-exercises-7.html"><i class="fa fa-check"></i><b>7.7</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="8" data-path="8-nrf.html"><a href="8-nrf.html"><i class="fa fa-check"></i><b>8</b> Nonlinear Regression Functions</a>
<ul>
<li class="chapter" data-level="8.1" data-path="8-1-a-general-strategy-for-modelling-nonlinear-regression-functions.html"><a href="8-1-a-general-strategy-for-modelling-nonlinear-regression-functions.html"><i class="fa fa-check"></i><b>8.1</b> A General Strategy for Modelling Nonlinear Regression Functions</a></li>
<li class="chapter" data-level="8.2" data-path="8-2-nfoasiv.html"><a href="8-2-nfoasiv.html"><i class="fa fa-check"></i><b>8.2</b> Nonlinear Functions of a Single Independent Variable</a>
<ul>
<li class="chapter" data-level="" data-path="8-2-nfoasiv.html"><a href="8-2-nfoasiv.html#polynomials"><i class="fa fa-check"></i>Polynomials</a></li>
<li class="chapter" data-level="" data-path="8-2-nfoasiv.html"><a href="8-2-nfoasiv.html#logarithms"><i class="fa fa-check"></i>Logarithms</a></li>
</ul></li>
<li class="chapter" data-level="8.3" data-path="8-3-interactions-between-independent-variables.html"><a href="8-3-interactions-between-independent-variables.html"><i class="fa fa-check"></i><b>8.3</b> Interactions Between Independent Variables</a></li>
<li class="chapter" data-level="8.4" data-path="8-4-nonlinear-effects-on-test-scores-of-the-student-teacher-ratio.html"><a href="8-4-nonlinear-effects-on-test-scores-of-the-student-teacher-ratio.html"><i class="fa fa-check"></i><b>8.4</b> Nonlinear Effects on Test Scores of the Student-Teacher Ratio</a></li>
<li class="chapter" data-level="8.5" data-path="8-5-exercises-8.html"><a href="8-5-exercises-8.html"><i class="fa fa-check"></i><b>8.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="9" data-path="9-asbomr.html"><a href="9-asbomr.html"><i class="fa fa-check"></i><b>9</b> Assessing Studies Based on Multiple Regression</a>
<ul>
<li class="chapter" data-level="9.1" data-path="9-1-internal-and-external-validity.html"><a href="9-1-internal-and-external-validity.html"><i class="fa fa-check"></i><b>9.1</b> Internal and External Validity</a></li>
<li class="chapter" data-level="9.2" data-path="9-2-ttivomra.html"><a href="9-2-ttivomra.html"><i class="fa fa-check"></i><b>9.2</b> Threats to Internal Validity of Multiple Regression Analysis</a></li>
<li class="chapter" data-level="9.3" data-path="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html"><i class="fa fa-check"></i><b>9.3</b> Internal and External Validity when the Regression is Used for Forecasting</a></li>
<li class="chapter" data-level="9.4" data-path="9-4-etsacs.html"><a href="9-4-etsacs.html"><i class="fa fa-check"></i><b>9.4</b> Example: Test Scores and Class Size</a></li>
<li class="chapter" data-level="9.5" data-path="9-5-exercises-9.html"><a href="9-5-exercises-9.html"><i class="fa fa-check"></i><b>9.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="10" data-path="10-rwpd.html"><a href="10-rwpd.html"><i class="fa fa-check"></i><b>10</b> Regression with Panel Data</a>
<ul>
<li class="chapter" data-level="10.1" data-path="10-1-panel-data.html"><a href="10-1-panel-data.html"><i class="fa fa-check"></i><b>10.1</b> Panel Data</a></li>
<li class="chapter" data-level="10.2" data-path="10-2-PDWTTP.html"><a href="10-2-PDWTTP.html"><i class="fa fa-check"></i><b>10.2</b> Panel Data with Two Time Periods: “Before and After” Comparisons</a></li>
<li class="chapter" data-level="10.3" data-path="10-3-fixed-effects-regression.html"><a href="10-3-fixed-effects-regression.html"><i class="fa fa-check"></i><b>10.3</b> Fixed Effects Regression</a>
<ul>
<li class="chapter" data-level="" data-path="10-3-fixed-effects-regression.html"><a href="10-3-fixed-effects-regression.html#estimation-and-inference"><i class="fa fa-check"></i>Estimation and Inference</a></li>
<li class="chapter" data-level="" data-path="10-3-fixed-effects-regression.html"><a href="10-3-fixed-effects-regression.html#application-to-traffic-deaths"><i class="fa fa-check"></i>Application to Traffic Deaths</a></li>
</ul></li>
<li class="chapter" data-level="10.4" data-path="10-4-regression-with-time-fixed-effects.html"><a href="10-4-regression-with-time-fixed-effects.html"><i class="fa fa-check"></i><b>10.4</b> Regression with Time Fixed Effects</a></li>
<li class="chapter" data-level="10.5" data-path="10-5-tferaaseffer.html"><a href="10-5-tferaaseffer.html"><i class="fa fa-check"></i><b>10.5</b> The Fixed Effects Regression Assumptions and Standard Errors for Fixed Effects Regression</a></li>
<li class="chapter" data-level="10.6" data-path="10-6-drunk-driving-laws-and-traffic-deaths.html"><a href="10-6-drunk-driving-laws-and-traffic-deaths.html"><i class="fa fa-check"></i><b>10.6</b> Drunk Driving Laws and Traffic Deaths</a></li>
<li class="chapter" data-level="10.7" data-path="10-7-exercises-10.html"><a href="10-7-exercises-10.html"><i class="fa fa-check"></i><b>10.7</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="11" data-path="11-rwabdv.html"><a href="11-rwabdv.html"><i class="fa fa-check"></i><b>11</b> Regression with a Binary Dependent Variable</a>
<ul>
<li class="chapter" data-level="11.1" data-path="11-1-binary-dependent-variables-and-the-linear-probability-model.html"><a href="11-1-binary-dependent-variables-and-the-linear-probability-model.html"><i class="fa fa-check"></i><b>11.1</b> Binary Dependent Variables and the Linear Probability Model</a></li>
<li class="chapter" data-level="11.2" data-path="11-2-palr.html"><a href="11-2-palr.html"><i class="fa fa-check"></i><b>11.2</b> Probit and Logit Regression</a>
<ul>
<li class="chapter" data-level="" data-path="11-2-palr.html"><a href="11-2-palr.html#probit-regression"><i class="fa fa-check"></i>Probit Regression</a></li>
<li class="chapter" data-level="" data-path="11-2-palr.html"><a href="11-2-palr.html#logit-regression"><i class="fa fa-check"></i>Logit Regression</a></li>
</ul></li>
<li class="chapter" data-level="11.3" data-path="11-3-estimation-and-inference-in-the-logit-and-probit-models.html"><a href="11-3-estimation-and-inference-in-the-logit-and-probit-models.html"><i class="fa fa-check"></i><b>11.3</b> Estimation and Inference in the Logit and Probit Models</a></li>
<li class="chapter" data-level="11.4" data-path="11-4-application-to-the-boston-hmda-data.html"><a href="11-4-application-to-the-boston-hmda-data.html"><i class="fa fa-check"></i><b>11.4</b> Application to the Boston HMDA Data</a></li>
<li class="chapter" data-level="11.5" data-path="11-5-exercises-11.html"><a href="11-5-exercises-11.html"><i class="fa fa-check"></i><b>11.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="12" data-path="12-ivr.html"><a href="12-ivr.html"><i class="fa fa-check"></i><b>12</b> Instrumental Variables Regression</a>
<ul>
<li class="chapter" data-level="12.1" data-path="12-1-TIVEWASRAASI.html"><a href="12-1-TIVEWASRAASI.html"><i class="fa fa-check"></i><b>12.1</b> The IV Estimator with a Single Regressor and a Single Instrument</a></li>
<li class="chapter" data-level="12.2" data-path="12-2-TGIVRM.html"><a href="12-2-TGIVRM.html"><i class="fa fa-check"></i><b>12.2</b> The General IV Regression Model</a></li>
<li class="chapter" data-level="12.3" data-path="12-3-civ.html"><a href="12-3-civ.html"><i class="fa fa-check"></i><b>12.3</b> Checking Instrument Validity</a></li>
<li class="chapter" data-level="12.4" data-path="12-4-attdfc.html"><a href="12-4-attdfc.html"><i class="fa fa-check"></i><b>12.4</b> Application to the Demand for Cigarettes</a></li>
<li class="chapter" data-level="12.5" data-path="12-5-where-do-valid-instruments-come-from.html"><a href="12-5-where-do-valid-instruments-come-from.html"><i class="fa fa-check"></i><b>12.5</b> Where Do Valid Instruments Come From?</a></li>
<li class="chapter" data-level="12.6" data-path="12-6-exercises-12.html"><a href="12-6-exercises-12.html"><i class="fa fa-check"></i><b>12.6</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="13" data-path="13-eaqe.html"><a href="13-eaqe.html"><i class="fa fa-check"></i><b>13</b> Experiments and Quasi-Experiments</a>
<ul>
<li class="chapter" data-level="13.1" data-path="13-1-poceaie.html"><a href="13-1-poceaie.html"><i class="fa fa-check"></i><b>13.1</b> Potential Outcomes, Causal Effects and Idealized Experiments</a></li>
<li class="chapter" data-level="13.2" data-path="13-2-threats-to-validity-of-experiments.html"><a href="13-2-threats-to-validity-of-experiments.html"><i class="fa fa-check"></i><b>13.2</b> Threats to Validity of Experiments</a></li>
<li class="chapter" data-level="13.3" data-path="13-3-experimental-estimates-of-the-effect-of-class-size-reductions.html"><a href="13-3-experimental-estimates-of-the-effect-of-class-size-reductions.html"><i class="fa fa-check"></i><b>13.3</b> Experimental Estimates of the Effect of Class Size Reductions</a>
<ul>
<li class="chapter" data-level="" data-path="13-3-experimental-estimates-of-the-effect-of-class-size-reductions.html"><a href="13-3-experimental-estimates-of-the-effect-of-class-size-reductions.html#experimental-design-and-the-data-set"><i class="fa fa-check"></i>Experimental Design and the Data Set</a></li>
<li class="chapter" data-level="" data-path="13-3-experimental-estimates-of-the-effect-of-class-size-reductions.html"><a href="13-3-experimental-estimates-of-the-effect-of-class-size-reductions.html#analysis-of-the-star-data"><i class="fa fa-check"></i>Analysis of the STAR Data</a></li>
</ul></li>
<li class="chapter" data-level="13.4" data-path="13-4-qe.html"><a href="13-4-qe.html"><i class="fa fa-check"></i><b>13.4</b> Quasi Experiments</a>
<ul>
<li class="chapter" data-level="" data-path="13-4-qe.html"><a href="13-4-qe.html#the-differences-in-differences-estimator"><i class="fa fa-check"></i>The Differences-in-Differences Estimator</a></li>
<li class="chapter" data-level="" data-path="13-4-qe.html"><a href="13-4-qe.html#regression-discontinuity-estimators"><i class="fa fa-check"></i>Regression Discontinuity Estimators</a></li>
</ul></li>
<li class="chapter" data-level="13.5" data-path="13-5-exercises-13.html"><a href="13-5-exercises-13.html"><i class="fa fa-check"></i><b>13.5</b> Exercises</a></li>
</ul></li>
<li class="chapter" data-level="14" data-path="14-ittsraf.html"><a href="14-ittsraf.html"><i class="fa fa-check"></i><b>14</b> Introduction to Time Series Regression and Forecasting</a>
<ul>
<li class="chapter" data-level="14.1" data-path="14-1-using-regression-models-for-forecasting.html"><a href="14-1-using-regression-models-for-forecasting.html"><i class="fa fa-check"></i><b>14.1</b> Using Regression Models for Forecasting</a></li>
<li class="chapter" data-level="14.2" data-path="14-2-tsdasc.html"><a href="14-2-tsdasc.html"><i class="fa fa-check"></i><b>14.2</b> Time Series Data and Serial Correlation</a>
<ul>
<li class="chapter" data-level="" data-path="14-2-tsdasc.html"><a href="14-2-tsdasc.html#notation-lags-differences-logarithms-and-growth-rates"><i class="fa fa-check"></i>Notation, Lags, Differences, Logarithms and Growth Rates</a></li>
</ul></li>
<li class="chapter" data-level="14.3" data-path="14-3-autoregressions.html"><a href="14-3-autoregressions.html"><i class="fa fa-check"></i><b>14.3</b> Autoregressions</a>
<ul>
<li><a href="14-3-autoregressions.html#autoregressive-models-of-order-p">Autoregressive Models of Order <span class="math inline">\(p\)</span></a></li>
</ul></li>
<li class="chapter" data-level="14.4" data-path="14-4-cybtmpi.html"><a href="14-4-cybtmpi.html"><i class="fa fa-check"></i><b>14.4</b> Can You Beat the Market? ( Part I)</a></li>
<li class="chapter" data-level="14.5" data-path="14-5-apatadlm.html"><a href="14-5-apatadlm.html"><i class="fa fa-check"></i><b>14.5</b> Additional Predictors and The ADL Model</a>
<ul>
<li class="chapter" data-level="" data-path="14-5-apatadlm.html"><a href="14-5-apatadlm.html#forecast-uncertainty-and-forecast-intervals"><i class="fa fa-check"></i>Forecast Uncertainty and Forecast Intervals</a></li>
</ul></li>
<li class="chapter" data-level="14.6" data-path="14-6-llsuic.html"><a href="14-6-llsuic.html"><i class="fa fa-check"></i><b>14.6</b> Lag Length Selection Using Information Criteria</a></li>
<li class="chapter" data-level="14.7" data-path="14-7-nit.html"><a href="14-7-nit.html"><i class="fa fa-check"></i><b>14.7</b> Nonstationarity I: Trends</a></li>
<li class="chapter" data-level="14.8" data-path="14-8-niib.html"><a href="14-8-niib.html"><i class="fa fa-check"></i><b>14.8</b> Nonstationarity II: Breaks</a></li>
<li class="chapter" data-level="14.9" data-path="14-9-can-you-beat-the-market-part-ii.html"><a href="14-9-can-you-beat-the-market-part-ii.html"><i class="fa fa-check"></i><b>14.9</b> Can You Beat the Market? ( Part II)</a></li>
</ul></li>
<li class="chapter" data-level="15" data-path="15-eodce.html"><a href="15-eodce.html"><i class="fa fa-check"></i><b>15</b> Estimation of Dynamic Causal Effects</a>
<ul>
<li class="chapter" data-level="15.1" data-path="15-1-the-orange-juice-data.html"><a href="15-1-the-orange-juice-data.html"><i class="fa fa-check"></i><b>15.1</b> The Orange Juice Data</a></li>
<li class="chapter" data-level="15.2" data-path="15-2-dynamic-causal-effects.html"><a href="15-2-dynamic-causal-effects.html"><i class="fa fa-check"></i><b>15.2</b> Dynamic Causal Effects</a></li>
<li class="chapter" data-level="15.3" data-path="15-3-dynamic-multipliers-and-cumulative-dynamic-multipliers.html"><a href="15-3-dynamic-multipliers-and-cumulative-dynamic-multipliers.html"><i class="fa fa-check"></i><b>15.3</b> Dynamic Multipliers and Cumulative Dynamic Multipliers</a></li>
<li class="chapter" data-level="15.4" data-path="15-4-hac-standard-errors.html"><a href="15-4-hac-standard-errors.html"><i class="fa fa-check"></i><b>15.4</b> HAC Standard Errors</a></li>
<li class="chapter" data-level="15.5" data-path="15-5-estimation-of-dynamic-causal-effects-with-strictly-exogeneous-regressors.html"><a href="15-5-estimation-of-dynamic-causal-effects-with-strictly-exogeneous-regressors.html"><i class="fa fa-check"></i><b>15.5</b> Estimation of Dynamic Causal Effects with Strictly Exogeneous Regressors</a></li>
<li class="chapter" data-level="15.6" data-path="15-6-orange-juice-prices-and-cold-weather.html"><a href="15-6-orange-juice-prices-and-cold-weather.html"><i class="fa fa-check"></i><b>15.6</b> Orange Juice Prices and Cold Weather</a></li>
</ul></li>
<li class="chapter" data-level="16" data-path="16-atitsr.html"><a href="16-atitsr.html"><i class="fa fa-check"></i><b>16</b> Additional Topics in Time Series Regression</a>
<ul>
<li class="chapter" data-level="16.1" data-path="16-1-vector-autoregressions.html"><a href="16-1-vector-autoregressions.html"><i class="fa fa-check"></i><b>16.1</b> Vector Autoregressions</a></li>
<li class="chapter" data-level="16.2" data-path="16-2-ooiatdfglsurt.html"><a href="16-2-ooiatdfglsurt.html"><i class="fa fa-check"></i><b>16.2</b> Orders of Integration and the DF-GLS Unit Root Test</a></li>
<li class="chapter" data-level="16.3" data-path="16-3-cointegration.html"><a href="16-3-cointegration.html"><i class="fa fa-check"></i><b>16.3</b> Cointegration</a></li>
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<div id="internal-and-external-validity-when-the-regression-is-used-for-forecasting" class="section level2" number="9.3">
<h2><span class="header-section-number">9.3</span> Internal and External Validity when the Regression is Used for Forecasting</h2>
<p>Recall the regression of test scores on the student-teacher ratio (<span class="math inline">\(STR\)</span>) performed in Chapter <a href="4-lrwor.html#lrwor">4</a>:</p>
<div class="sourceCode" id="cb226"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb226-1"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-1" aria-hidden="true"></a>linear_model &lt;-<span class="st"> </span><span class="kw">lm</span>(score <span class="op">~</span><span class="st"> </span>STR, <span class="dt">data =</span> CASchools)</span>
<span id="cb226-2"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-2" aria-hidden="true"></a>linear_model</span>
<span id="cb226-3"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-3" aria-hidden="true"></a><span class="co">#&gt; </span></span>
<span id="cb226-4"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-4" aria-hidden="true"></a><span class="co">#&gt; Call:</span></span>
<span id="cb226-5"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-5" aria-hidden="true"></a><span class="co">#&gt; lm(formula = score ~ STR, data = CASchools)</span></span>
<span id="cb226-6"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-6" aria-hidden="true"></a><span class="co">#&gt; </span></span>
<span id="cb226-7"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-7" aria-hidden="true"></a><span class="co">#&gt; Coefficients:</span></span>
<span id="cb226-8"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-8" aria-hidden="true"></a><span class="co">#&gt; (Intercept)          STR  </span></span>
<span id="cb226-9"><a href="9-3-internal-and-external-validity-when-the-regression-is-used-for-forecasting.html#cb226-9" aria-hidden="true"></a><span class="co">#&gt;      698.93        -2.28</span></span></code></pre></div>
<p>The estimated regression function was</p>
<p><span class="math display">\[ \widehat{TestScore} = 698.9 - 2.28 \times STR.\]</span></p>
<p>The book discusses the example of a parent moving to a metropolitan area who plans to choose where to live based on the quality of local schools: a school district’s average test score is an adequate measure for the quality.  However, the parent has information on the student-teacher ratio only such that test scores need to be predicted.  Although we have established that there is omitted variable bias in this model due to omission of variables like student learning opportunities outside school, the share of English learners and so on, <tt>linear_model</tt> may in fact be useful for the parent:</p>
<p>The parent need not care if the coefficient on <span class="math inline">\(STR\)</span> has causal interpretation, she wants <span class="math inline">\(STR\)</span> to explain as much variation in test scores as possible.  Therefore, despite the fact that <tt>linear_model</tt> cannot be used to estimate the <em>causal effect</em> of a change in <span class="math inline">\(STR\)</span> on test scores, it can be considered a <em>reliable predictor</em> of test scores in general.</p>
<p>Thus, the threats to internal validity as summarized in Key Concept 9.7 are negligible for the parent.  This is, as instanced in the book, different for a superintendent who has been tasked to take measures that increase test scores: she requires a more reliable model that does not suffer from the threats listed in Key Concept 9.7.</p>
<p>Consult Chapter 9.3 of the book for the corresponding discussion.</p>
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