In a regression analysis involving 30 observations, the following estimated regression equation was obtained. yhat = 17.6 + 3.8x1 - 2.3x2 + 7.6x3 + 2.7x4 For this estimated regression equation SST = 1805 and SSR = 1760 a) - At alpha = 0.05, test the significance of the relationship among the variables. Suppose variables x1 and x4 are dropped from the model and the following estimated regression equation is obtained. yhat = 11.1 - 3.6x2 + 8.1x3 For this model, SST = 1805 and SSR = 1705 b) Compute SSE - (x1, x2, x3, x4) c) Compute SSE - (x2, x3) d) Use an F test and a .05 level of significance to determine whether x1 and x4 contribute significantly to this model. For part b), c), d), please use the "adding variables F-Test"