Deterministic models predict an exact outcome, given certain initial conditions.
Stochastic models predict variable outcomes based on probabilities of occurrence.
Types of Stochasticity Demographic stochasticity - variation in demographic rates among individuals. Demographic stochasticity often arises because of variation in sampling of individuals. An extremely skewed sex ratio also is an example of an unusual demographic event. This stochasticity occurs even in a constant environment. In relatively small populations, demographic stochasticity can be an important factor influencing extinction probability.