Sequentially mixing modes in an election survey

In this paper, we analyze to what extent a sequential mixed mode survey, consisting of push-to-web, telephone and mail-paper modes, is able to improve representativeness in terms of socio-demographic variables and reduce bias in terms of voting behavior compared to a single mode survey. In addition, we study whether changes in mode lead to measurement error issues by focusing on income. We find that adding the telephone mode improves sample representation in terms of socio-demographic variables. Meanwhile, adding the paper mode does not show further improvements in this respect. However, adding the telephone, and in particular the paper mode, turn out to reduce bias in voting behavior when compared to official figures. As for measuring income, the web and the telephone mode perform similarly well. Finally, we find little evidence of measurement differences of income when the same respondents are interviewed first by web or telephone and subsequently by paper questionnaire