Non-stoichiometry in ternary oxide perovskites is analyzed to examine its donor/acceptor-like behavior using the doping factor concept. The general form, A+ α B+ β O3, represents the three ternary oxide perovskite systems: III-III, II-IV and I-V. The five most important native point defects in those systems are dealt with, namely, A- and B- metal vacancies, oxygen vacancies, electrons and holes. The influence of non-stoichiometry on the electron and hole concentrations compared to a reference state can be described by expansion of the doping factor concept from binary to ternary systems. The doping factor, f, is the parameter that quantifies the concentration change of quasi-free electrons in a solid upon changes in composition. In particular, f > 1 when the solid is doped with donors and f < 1 when it is doped with acceptors. In the ternary system, changes in the A/B ratio in the undoped material are accompanied by small deviations of the metal vacancy concentrations, and therefore result in deviation of the doping factor f from unity. This deviation is expressed using an additional doping factor, which results from the added degree of freedom in the ternary case as compared to the binary system. It is shown that despite the well-known fact that frozen-in metal vacancies behave as acceptors, in the perovskite systems II-IV and I-V, there is a finite range in the A/B ratio where an increase in metal vacancy concentration results in donor-like behavior. In addition to its theoretical aspects, the importance of this analysis stems from the fact that, in practice, small deviations from stoichiometry in the cation sub-lattice are inevitable. Understanding the way in which these materials respond to non-stoichiometry is crucial for comprehending the observed electrical phenomena.