  In a party-based election system, the voters are grouped into parties and all
voters of a party are assumed to vote according to the party preferences over
the candidates. Hence, once the party preferences are declared the outcome of
the election can be determined. However, in the actual election, the members of
some "instable" parties often leave their own party to join other parties. We
introduce two parameters to measure the credibility of the prediction based on
party preferences: Min is the minimum number of voters leaving the instable
parties such that the prediction is no longer true, while Max is the maximum
number of voters leaving the instable parties such that the prediction remains
valid. Concerning the complexity of computing Min and Max, we consider both
positional scoring rules (Plurality, Veto, r-Approval and Borda) and
Condorcet-consistent rules (Copeland and Maximin). We show that for all
considered scoring rules, Min is polynomial-time computable, while it is
NP-hard to compute Min for Copeland and Maximin. With the only exception of
Borda, Max can be computed in polynomial time for other scoring rules. We have
NP-hardness results for the computation of Max under Borda, Maximin and
Copeland.
