In order to obtain unbiased estimates from survey interviews, it is important that the data is of good quality: i.e. representativity of the survey respondents and variables that are free of measurement error. Using administrative records and survey data, the main questions we address concern the differential nonresponse bias between the telephone and the Web mode and whether these modes lead to differential measurement error. In an experimental setting we randomly assigned respondents to either phone (n=2,400) or Web mode (n=1,082). Because the sampled persons were selected from German administrative records, record data are available for all sample units to study the bias due to nonresponse and measurement error (e.g. population means and regression parameters). Hence, we can assess the overall nonresponse bias of the estimates by comparing the statistics from both modes against the known population value. Similarly, for the respondents, we compare survey values and administrative records on individual level for selected variables and compute measurement error directly. First, based on administrative data for respondents and nonrespondents, our paper compares nonresponse bias for the above statistics in the single telephone mode to those obtained in the Web mode. Empirical analyses confirm a differential sample composition resulting in systematically different nonresponse bias between the two modes. Second, we assess the amount of measurement error for both modes. We conclude with a discussion of whether mode specific differences, with respect to nonresponse bias and measurement error bias, compensate or reinforce each other with respect to the total error.