Party System Innovation: overall vote share of new parties in a given election. It is calculated at time t with respect to time t-1 (namely, PSInn is calculated with respect to the status quo established at the previous election) and therefore each observation in each country is completely independent from the previous ones. In order to exclude marginal parties, the author has set a threshold at 1% of the national share for a given party to be considered as part of the party system in a given election and has collected data starting from the third post-World War II or democratic election of each country, for a total of 209 new parties (see the complete list of new parties below) in 327 elections. The underlying assumption is that the party system innovation they are interested in is that occurring after the initial institutionalization of the party system. According to PSInn, a party is considered 'new' only in the first election when it enters the party system by receiving at least 1% of the national share. Then, in the subsequent elections, it becomes 'old'.