Survey researchers inevitably face the problem of item non-response; that is, some respondents provide valid answers to some questions, but provide invalid answers to others, such as “don’t know,” “no opinion,” or “hard to say.” However, including only those respondents who provide valid answers for each question may bias the estimates, because non-response does not occur randomly.

Previous studies on voting participation used to model individuals’ utilities and duties as the forces driving their participation. It is assumed that people maximize their utilities when they turn out. In the 2004 referendum, there were many controversies as to how to exercise direct democracy as well as fierce partisan battles. It is thus necessary to analyze the determinants of voting in the referendum from the perspective of social psychology. There are few empirical studies on the 2004 referendum.