In the standard model the parameters {rho} and M{sub W}{sup 2}/M{sub Z}{sup 2}cos{sup 2}{theta} are unity at tree level. They are protected against large corrections by the so-called custodial symmetry. We investigate the symmetry breaking effect of effect of mass splittings in fermion doublets. The effective field theory method is applied to sum up the leading-log QCD corrections. We resolve a controversy concerning the scale at which the strong coupling constant is to be evaluated in the two-loop correction to electorweak parameters.