Decomposing fluctuations in the output of a biochemical system. In this example, we consider how fluctuations in three variables, denoted Yi, affect fluctuations in Z, the system’s output. Y1 is a biochemical species within the system being studied; Y2 and Y3 are variables in other stochastic systems that interact with the system of interest, but whose dynamics are principally generated independently of that system. We show Y2 and Y3 in the same system, but they need not be.