 It is common for some factors within a causal system to be dependent for their value in period "t" on the values of other factors in the causal system in period "t" − 1.Suppose that the level of pest infestation is independent of all other factors within a given period, but is influenced by the level of rainfall and fertilizer in the preceding period.In this instance it would be correct to say that infestation is exogenous within the period, but endogenous over time.