# Loop Impact

### Impact

The impact of one stock on another is a measure of how changes in a source stock cause changes in the motion of the target stock.

Let Y be a source stock influencing a target X. A change in Y will cause X to accelerate or decelerate, figure 1. This is the impact of Y on X.

The impact of Y on X is measured by the acceleration imparted to X by Y, divided by the rate of change of X. Thus impact is a ratio that measures the curvature in the graph of X over time, figure 2. Constant impact would give an exponential curve.

### Loop Impact – First-Order Loops

Loop impact is the impact of the feedback loop on the behaviour of each stock in the loop. Specifically, this is the impact on the stock by the previous stock in the loop. Look at figure 3. The stock N directly influences itself through two loops, R and B. Each loop has an impact, in this case, b and -d. The minus sign represents a balancing loop. In each case, N is both the source stock and its own target stock.