Cohesion of Parliamentary Groups in Polish Sejm (4th, 5th, 6th and 7th Terms)

Group cohesion in collective decision making can be described as the degree in which the group's members concur in the positions they take. When a collective body makes decisions by voting, this cohesion can be examined by means of observation of the behavior of its members in the roll call votes, and quantified. Such quantitative description of cohesion can be used to:

  • compare the cohesion of different groups
  • study the changes in cohesion of a group over time
  • study the relation between the cohesion of a given group and the conflict level within a decision-making body
  • study of factors determining group cohesion

As far as lawmaking bodies are concerned, parliamentary groups are the most usual subject of analysis. It is, however, possible to examine the cohesion of groups of lawmakers from a given territory, or (in parliamentary systems) the cohesion of government coalition and parliamentary opposition.

In our analyses of parliamentary groups we employ the Hix index (a modified version of Rice index), denoted by the letters CI (cohesion index). It can be used in situations in which each member of a decision-making body participating in a vote can make one of the three decisions:

  • vote against the decision (nay - N);
  • vote for the decision (yea - Y);
  • abstain from voting (abstain - A).

CI is computed using the following formula:

CI := 1 - 3/2 * D

where:

D := 1 - max(Y, N, A).

and:

  • Y – number of group members voting for the decision;
  • N – number of group members voting against the decision;
  • A – number of group members who abstain from voting;
  • y = Y / (N + A + Y) – fraction of group members voting for the decision;
  • n = N / (N + A + Y) – fraction of group members voting against the decision;
  • a = A / (N + A + Y) – fraction of group members who abstain from voting;

 

The research is primarily aimed at describing the collective behavior of the Sejm (the lower house of Polish Parliament) factions during the 4th, 5th, 6th, and 7th Terms. The gathered data will be used for further analyses, including the study of:

  • changes of cohesion in time and their possible causes
  • relation between the parliamentary group cohesion and its voting power, calculated according to the Banzhaf index
  • relation between the parliamentary group cohesion and its size and participation in the governing coalition
  • relation between the parliamentary group cohesion and the polarization of political conflict in Sejm