Clarifying the Measurement of Relative Ideology in Policy Diffusion Research

Research on policy diffusion has recently paid more attention to ideological patterns of policy adoption. Grossback, Nicholson-Crotty, and Peterson operationalized a measure of ideological diffusion; however, it has not been consistently calculated in subsequent studies. This is mainly due to difficulties in interpreting how to measure ideological distance based solely on the original article. Specifically, there are three factors that prevent common measurement of the concept: starting values, adoption ties, and weighting of recent adoptions. Recommendations are made for each of these. The purpose is to establish a consistent ideological distance measure. To illustrate, a replication of the original lottery diffusion model in the authors’ paper shows how the results change with different measurement choices. Consistently measuring this concept is important as scholars increasingly recognize that states do not always follow their geographic neighbors but increasingly their ideological “neighbors.”