25. If these data are not from a randomized experiment, there are many sources of confounding that could explain the differences.
27. Combine them to compare intercepts and compare the slopes.
29. The residuals match.
31. Unless the slopes are known to be equal, consider an interaction. The slope estimates the cost per hour of labor. We expect it to be higher in the union shop.
The intercept is the mean salary for women($140,034); the slope is the difference in salaries, with men making $4,671 more than women.
The variances are assumed equal.
The slope will be closer to zero.
Yes, the slope of the interaction (D x) is not statistically significant.
The coefficients of D and x D would both become ...