The strength of bases is determined by the pKa values of their conjugate acids. This relationship arises from the inverse relationship between the strengths of acids and their conjugate bases: stronger acids have weaker conjugate bases, and vice versa. Consequently, the higher the pKa value of a conjugate acid, the stronger the corresponding base. In simpler terms, equilibrium tends to favor the side with the weaker base, which corresponds to the side with the higher pKa value of its conjugate acid.