How and why does the mode of data collection affect consent to data linkage?

We use experimental mixed-mode data from a probability survey in Great Britain to examine why respondents are less likely to consent to data linkage in online than face-to-face interviews. We find that the 30 percentage point difference in consent rates is a causal effect of the mode on willingness to consent; it is not due to selection of different types of respondents into web and face-to-face interviews. We find that respondents are less likely to understand the data linkage request, less likely to process the consent request thoroughly, and more likely to be concerned about privacy and data security when answering online rather than in a face-to-face interview. Using digital audio-recordings of the face-to-face interviews, we find that verbal behaviours of interviewers do not explain the mode effects: respondents only rarely ask questions or express concern, and interviewers only rarely offer additional information about the data linkage. We also examine which devices respondents used to complete the web survey and find that these do not explain the mode effects either. Finally, we test the effects of simplifying the consent request, by reducing the reading difficulty: while the easier wording increases understanding of the request, it does not increase consent in either mode