This chapter describes the source and nature of the data to be used for the empirical test of the dynamic model. Ideally, the dynamic model would be subjected to rigorous empirical tests using both cross-national and longitudinal data from all Western industrial democracies. There are several data on party professionalization that are currently available. Cross-national comparisons also show the effect of the electoral system on organizational complexity. Party membership size continues to exert significant influence on the centralization of power in a political party. The evidence suggests that professionalization is significantly related to intra-party democratic performance. Despite the fact that increasing levels of party professionalization may lead to greater centralization of power, the size of party membership puts certain limits to the degree of decentralization of power in a party. Data for the variable "organizational complexity" were operationalized and measured using data obtained from the Robert Harmel and Kenneth Janda project.