  The Onsager-de Groot-Callen transport theory, implemented as a network model,
is used to simulate the transient Harman method, which is widely used
experimentally to determine all thermoelectric transport coefficients in a
single measurement setup. It is shown that this method systematically
overestimates the Seebeck coefficient for samples composed of two different
materials. As a consequence, the figure of merit is also overestimated, if the
thermal coupling of the measurement setup to the environment is weak. For a
mixture of metal and semiconductor particles near metal percolation the figure
of merit obtained by the Harman method is more than 100 \% too large. For a
correct interpretation of the experimental data, information on composition and
microstructure of the sample are indispensable.
