A model for disaggregating political change

We present a model for identifying the components of aggregate change in an electorate between two points in time. When the electorate is constant in size, change in the mean opinion disaggregates into two components: conversion among those who continue to participate in the electorate (Stayers), and replacement of those who drop out of the electorate at Time₁ (Dropouts) by Newcomers to the electorate at Time₂. We add to this simple formulation the possibility of variation in the size of the electorate. When an electorate expands, the model includes a mobilization term to accommodate the fact that there are more Newcomers at Time₂ than Dropouts at Time₁. When an electorate shrinks in size, the demobilization term reflects the fact that Newcomers as a group are smaller than Dropouts. The model includes appropriate weights for each component so that the change in opinion (or any other aggregate characteristic in the electorate) can be allocated across the three components. We apply the model to Iowa caucus attenders in both parties between 1984 and 1988, and we suggest that the model can be also productively applied to a variety of contexts besides nomination politics, where fluctuations in the size of electorates are significant.Ronald B. Rapoport. College of William and Mary.