  The evolution of resistivity versus temperature ($\rho(T)$) curve for
different doping elements, and in the presence of various defects and
clustering are explained for both diluted magnetic semiconductors (DMS) and
manganites. Here, we provide unambiguous evidence that the concept of
ionization energy ($E_I$), which is explicitly associated with the atomic
energy levels, can be related quantitatively to transport measurements. The
proposed ionization energy model is used to understand how the valence states
of ions affect the evolution of $\rho(T)$ curves for different doping elements.
We also explain how the $\rho(T)$ curves evolve in the presence of, and in the
absence of defects and clustering. The model also complements the results
obtained from first-principles calculations.
