3. Question 4 In studying how class size affected the test scores for different school classes, we obtained very different results depending on whether or not we included other variables. 4.1 The effect of class size on test scores is less than half as large when including the share of English learners in the class. How can this be explained in terms of omitted variable bias? (4 points) 4.2 What is omitted variable bias? Under what conditions is it problematic to omit a variable? (4 points) 4.3 If classes in wealthier areas are smaller than in poorer ones, how do you think the inclusion or omission of income affects the estimate of class size on test scores? What factors would determine if this effect is large or small? (5 points)