There is no unique definition of what is a sufficient population for the continuation of a species, because whether a species survives will depend to some extent on random events. Thus any calculation of a minimum viable population (MVP) will depend on the population projection model used.[3] A set of random (stochastic) projections might be used to estimate the initial population size needed (based on the assumptions in the model) for there to be (say) a 95% or 99% probability of survival say 1,000 years into the future.[4] Some models use generations as a unit of time rather than years, in order to maintain consistency between taxa.[5] These projections (population viability analyses, or PVA) use computer simulations to model populations using demographic and environmental information to project future population dynamics. The probability assigned to a PVA is arrived at after repeating the environmental simulation thousands of times.