  Several application domains require formal but flexible approaches to the
comparison problem. Different process models that cannot be related by
behavioral equivalences should be compared via a quantitative notion of
similarity, which is usually achieved through approximation of some
equivalence. While in the literature the classical equivalence subject to
approximation is bisimulation, in this paper we propose a novel approach based
on testing equivalence. As a step towards flexibility and usability, we study
different relaxations taking into account orthogonal aspects of the process
observations: execution time, event probability, and observed behavior. In this
unifying framework, both interpretation of the measures and decidability of the
verification algorithms are discussed.
