A political scientist is studying a regression model with the usual assumptions, including IID errors. The design matrix X is fixed, with full rank p = 5, and n = 57. The chief parameter of interest is β2 −β4. One possible estimator is βˆ

2 −βˆ

4, where βˆ = (X

X)−1X

Y . Is there another linear unbiased estimator with smaller variance? Explain briefly.