Your Textbook Extends the Simple Regression Analysis of Chapters 4 $\widehat { \text { TestScore } } = 698.9 - 2.28 \times \text { STR }$

Your textbook extends the simple regression analysis of Chapters 4 and 5 by adding an
additional explanatory variable, the percent of English learners in school districts (PctEl).
The results are as follows: $\widehat { \text { TestScore } } = 698.9 - 2.28 \times \text { STR }$
$\text {and}$
$\widehat { \text { TestScore } } = 686.0 - 1.10 \times S T R - 0.65 \times \text { PctEL }$ Explain why you think the coefficient on the student-teacher ratio has changed so
dramatically (been more than halved).