**As the effect of a causal link (or loop) on a stock gets smaller, then the measure used for that causal link should tend to zero**.

This test ensures continuity between the model with casual link or loop and the model without.

This test is illustrated with the three loop limits-to-growth model, figure 1. The stock *x* is subject to a growth mechanism, loop* R0*. The effects of this growth are limited by capacity *M*, loop *B1*. The third loop is a drain from x, loop B0, which causes the stock to fall short of capacity

The loop gain of *B0* is *b.* Figure 2 shows the stock behaviour as *b *is reduced from 0.05 to zero. The trajectory of x gets closer to that of the system with loop *B0* removed, that is *b *= 0.

In this model, the impact of B0 is just the loop gain b. Thus, loop impact satisfies this continuity test in this instance. The percentage of the total impact of the three loops can also satisfies the continuity test. Figure 3 shows the percentage impact of *B0* tending to zero as loop gain tends to zero:

The way impact is constructed it will satisfy the continuity test in any circumstances where weakening a loop of link. For example, in the model of figure 1, letting *M* tend to infinity would produced behaviour that tends to the linear model, with just *R0* and *B0*. Correspondingly, the impact of *R0*, I*(R0) = a*(1-*x*/*M*) -> *a*. Likewise, the impact of *B1* = –*ax*/*M *-> 0. The analytical expressions of loop impact in this model are derived in Hayward & Roach 2017 and in: