Step 1 2 3 Constant 1.438 1.467 1.487 Wall Contact- Con -0.070 -0.072 -0.073 T-Value -1.34 -1.38 -1.40 P-Value 0.182 0.168 0.162 Level- L -0.048 -0.055   T-Value -1.35 -1.52   P-Value 0.178 0.131   Wall's material- W -0.040     T-Value -1.09     P-Value 0.276     S 0.337 0.336 0.336 R2 1.09 2.20 2.92 0.48 0.99 1.11 MCP -1.2 -1.0 -0.2

Table 7: Stepwise Regression: log (C) versus all factors. Value of Alpha-to- Enter was 0,5 and of Alpha-to-Remove 0,5. P-values represent calculated error probabilities and T-values, the corresponding values of t-student's test for the comparison of a P-value with a. The constants of the linear fit at each step are shown in first row. Bold values represent the corresponding slopes of each factor at each step. MCP is the Mallow's Cp.