Which measure of political difference between parties works better? A comparison in predicting coalition formation

The majority of measures of party politics have used the manifesto dataset as their source. Most of them have been proposed as alternatives to the RILE index, but the latter still continues to be the most often used and criticised index. All of such measures have been justified on theoretical and methodological grounds, as well as through appeals to face and convergent validity. However, we do not have information on how they actually compare in modelling reality. This paper provides such a comparison in a context where there is extensive evidence that political differences between parties matter for party interaction - coalition formation. The comparison focuses on contrasting a direct measure of political difference, the index of similarity, to measures based on ideological positions, all of which are derived from the manifesto data set. The analysis shows that the much simpler index of similarity performs just as well or better.