=+ Here is the Minitab output from fitting the model with the aforementioned independent variables as predictors (also fit by the authors of the cited article):

Predictor Coef SE Coef T p Constant 1507.3 206.8 7.29 0.000 x1 -4.282 4.969 -0.86 0.407 x2 7.46 62.11 0.12 0.907 x3 -0.9162 0.6227 -1.47 0.169 x4 90.60 24.84 3.65 0.004 s = 4.6885 R-sq = 59.6% R-sq(adj) = 44.9%

SOURCE DF SS MS F p Regression 4 40048 10012 4.06 0.029 Res.Error 11 27158 2469 Total 15 67206