Kramers-Kronig transforms may be used directly to verify the consistency of electrochemical impedance data obtained using the harmonic method, when the system is causal, stable, linear and time invariant, and finite as omega --> 0 and omega --> infinity. Using Kramers-Kronig transforms is more complicated when one of these conditions is not fulfilled. The coherence function can, on the other hand, always be used to verify the validity of experimental impedance measurements obtained using the stochastic method.