Mixed mode surveys are prone to confounding mode effects with nonresponse error. Sequential mixed mode designs, for instance, follow up nonrespondents to one mode with a subsequent mode (Dillman, 2000). The sample composition consequently could differ to a large extent across survey modes, hampering straightforward conclusions about mode effects. But even in experimental mixed mode survey designs, such a confounding can occur if nonresponse is not controlled. As long as different survey modes attract different types of respondents, sample comparability is compromised, and it will be difficult to filter out mode effects (Voogt & Saris, 2005).