(a) Correlation coefficient between [E]_n and [T_a], [T_d] of n − 3 to n + 2 SCs (total 12 parameters) is shown for w_s = 9, 11, and 13. (b) Correlation coefficient between [E]_n and test parameter [T_test]_n is shown with corresponding number of variables used for estimating the [T_test]_n. The combination C₁ shows a correlation coefficient almost identical to that shown by the other five test parameters, but with the least number of variables. This optimum combination is preferred over the other five combinations.