Our study investigates the differences in answering patterns (especially “don’t know”) between telephone survey and Web survey. We varied the presentation of DK and applied scales for two possible responsible concepts. At first we adapt a scale for measuring Social Desirability tendencies by Winkler, Kroh & Spiess (2006) and secondly a shortened scale to measure the Need for Cognition of each respondent by Bless et al. (1994). Both concepts could lead to different answering patterns in terms of item nonresponse but beyond that also to more general measurement error. We found small differences regarding SDR and NFC between the respondents of the two modes. SDR causes effects only in CATI survey and NFC causes effects only in Web survey.