The loop impact method was primarily designed for examining the impact of loops on stocks. Stocks are the dynamic variables in a system dynamics model because they accumulate, or integrate, material over time. Impact describes the effect of one stock on another, even if the causal link is not part of a loop. Impact also describes the effect of exogenous inputs on a stock.

However, it is also possible to examine the impact of a loop on a flow. As there must be at least one stock in a loop, it follows that there must be at least one flow in the loop. Impact involves the second derivative of a quantity. Thus, the impact of a flow relates to its second derivative. As flows are the first derivative of a stock, the impact of a flow uses the third derivative of a stock.

An example of this process is given in the impact of the stocks on the flow “sales” in the Bass diffusion/SI model: Force Flow