One of the methodological challenges faced by interest group scholars has been to construct an overview of the population of interest groups as a whole. A lack of registers has made it difficult to map the relevant interest group population. Moreover, even if recent years have witnessed an increase in large n studies of interest groups and studies of interest group populations, we still have few population sources of a time series nature which allow us to track developments in interest group populations over time. To fill this gap, our study relies on a new and unique dataset of the Interest group population active in lobbying the European Parliament in the period 2005 to 2010 coded by the European-wide INTEREURO project. Using a hierarchical Bayesian modelling approach we examine what explains the variation in the number and diversity of interest representatives from the different EU member states across space and time from 2005 to 2010. Even if it is widely accepted that interest representatives play a prominent role in the EU construction, we have limited knowledge as to whether the representation of interest representatives from different member states mirrors or modifies the balance of power between member states in the formal EU bodies. Our models test whether interest group representation from a country is affected by a series of factors related to its vote share in the formal EU bodies, background demographics, character of political system, economic degree of prosperity, volume of EU trade, and attitude towards European integration.
