=+41. The article “The Undrained Strength of Some Thawed Permafrost Soils” (Canadian Geotechnical J., 1979: 420–427) reported the following data on undrained shear strength of sandy soil (y, in kPa), depth (x1, in m), and water content (x2, in %):

Obs Depth Watcont Shstren 1 8.9 31.5 14.7 2 36.6 27.0 48.0 3 36.8 25.9 25.6 4 6.1 39.1 10.0 5 6.9 39.2 16.0 6 6.9 38.3 16.8 7 7.3 33.9 20.7 8 8.4 33.8 38.8 9 6.5 27.9 16.9 10 8.0 33.1 27.0 11 4.5 26.3 16.0 12 9.9 37.8 24.9 13 2.9 34.6 7.3 14 2.0 36.4 12.8 Fitting the model with predictors x1 and x2 only gave SSResid 5 894.95, whereas fitting the complete second-order model with predictors x1, x2, x 2

1, x 2