highly significantp < 0.0001
The frequency of positive sites denoted 15 % in 1993, 37 % in 1998, and 88 % in 2007, and the difference was highly significant (Chi-square test, χ 2 = 163.17; degrees of freedom ( df ) = 2; p < 0.0001).
The frequency of positive sites denoted 15 % in 1993, 37 % in 1998, and 88 % in 2007, and the difference was highly significant (Chi-square test, χ 2 = 163.17; degrees of freedom ( df ) = 2; p < 0.0001).