A reexamination is made of previous resistivity data of the TiO0.80-1.23 metallic oxide system, in which very large positive and negative temperature coefficients of resistivity α are observed. TiO0.80-1.23 samples contain a large number of stoichiometric vacancies which can be thermally ordered and disordered, drastically changing the residual resistivity, without seriously affecting the superconducting Tc. Fitting the results to the two-level model shows that any resistivity due to the two-level system must operate in parallel with the normal electron-phonon interactions. A phenomenological formula, based on the ideas of incipient localization in very-high-resistivity materials, is introduced. This formula fits all the data for the TiO0.80-1.23 system in a semiquantitative way. It also predicts that a zero α (the Mooij criterion) occurs when the elastic mean free path and the weak localization coherence length are approximately equal. © 1982 The American Physical Society.