The paper analyses the efforts of a party group leadership to provide a cohesive voting behaviour in their party groups due to discipline. Party group leaders are interested in cohesion in order to maximise their voting power, to maintain their credibility towards voters and to protect their party label (Mitchell, 1999). Most studies on party group cohesion concentrate on institutional variables (Carey, 2007; Depauw, 2003) such as the governmental system, the election system or on ideological heterogeneity in party groups. They assume party group disciplinary measures as given and do not measure them explicitly. This study presents a new measurement of party group discipline based on semi-qualitative interview data with 77 party group leaders and party experts collected in five European parliamentary systems (United Kingdom, Germany, European Parliament, Netherlands, Switzerland) in 2007-08. The suggested index encompasses not only institutional resources to punish and reward party group members (Sieberer, 2006), but also more fine-grained and behavioural means. The second part of the paper explains the variance of the index by analysing whether the government status, the size of parties, certain partisan ideologies of parties or institutional factors explain the degree of party group disciplinary measures in the parliaments under investigation.
