  The most realistic model for current-to-voltage measurements of electrical
impedance tomography is the complete electrode model which takes into account
electrode shapes and contact impedances at the electrode/object interfaces.
When contact impedances are small, numerical instability can be avoided by
replacing the complete model with the shunt model in which perfect contacts,
that is zero contact impedances, are assumed. In the present work we show that
using the shunt model causes only a (almost) linear error with respect to the
contact impedances in modelling absolute current-to-voltage measurements.
Moreover, we note that the electric potentials predicted by the two models
exhibit genuinely different Sobolev regularity properties. This, in particular,
causes different convergence rates for finite element approximation of the
potentials. The theoretical results are backed up by two dimensional numerical
experiments.
