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Spark plasma sintering (SPS) is a convenient and rapid means of producing dense ceramic compacts. However, the mechanisms responsible for rapid densification have not been identified satisfactorily, with different studies using an indirect approach yielding varied values for the densification parameters. This study involved SPS in high purity nanocrystalline alumina with temperatures ranging from 1173 to 1423 K and stresses from 25 to 100 MPa. A direct approach, with analyses at a constant density, revealed a stress exponent of ~1 and an inverse grain size dependence of ~3, consistent with Coble creep process. Whereas the direct approach gives a stress exponent of ~1, the indirect approach used previously gives stress exponents ranging from ~2.2 to 3.5 with the same data, thereby revealing potentially spurious values of the densification parameters from conventional indirect approaches to characterizing densification. The rapid densification during SPS is related to the finer grain sizes retained with the rapid heating rates and the imposed stress that enhances the driving force for densification.