Peter Hedström
Bearing in mind the illustrative nature of these analyses, what do these parameter values tell us? First of all, it shows that the “effective transition parameter” was about 30 times as high for the outflow as for the inflow (b = .009 and a = .0003). Most of this difference is likely to reflect differences in network densities. Since this was a rather small movement it seems likely that most exmembers and members knew one another. This means that the density of the network linking past and current members is likely to have been close to unity, and the b-parameter therefore directly reflects how effective the ex-members were in swaying members to leave the movement. A parameter value of .009 hence suggests that approximately 1 out of 100 ties between ex-members and members led to a member leaving the movement each year. If we assume that the members were equally effective in their interactions with potential members, this suggests that the density of ties between members and potential members was about .03 (≈ .0003/.009). Furthermore, the δ-estimate of .68 suggests that, on average, ex-members entered the ranks of the potentially recruitable about 1.5 years after they left the movement (.68–1 = 1.47). Models like these can be useful for performing what-if analyses. What would, for instance, be likely to happen with the long-term size of the movement if the networks linking potential, current, and past members to one another were altered? Inserting the estimated parameters into Equation 1, one finds that the implied long-term equilibrium size of the Husby-Rekarne movement was about 67 members. By changing the size of the networkrelated parameters we can get some sense of how changes in network densities are likely to affect the size of the movement (see Figure 4).
Change in Equilibrium Size
20
10
0
-10
a b c
δ
-20 -25
0 10 -10 Parameter Change %
25
Figure 4. Predicted changes in the equilibrium size of the movement due to changes in parameter values. – 122 –