  We reconsider the choice of renormalization schemes in a
differential-equation approach to aid the discussion of the renormalization of
the unstable particles and the CKM matrix in the Standard Model. Certain mass
dependent schemes do not satisfy these natural differential equations modulo
trace anomaly. By the way, the Callan-Symanzik equations were employed to show
that both mass dependent and independent schemes realize fermion decoupling in
the same way.
