It was observed that more and more respondents complete web surveys via mobile devices. However, we can expect an effect of the device used to complete the survey (PC or mobile devices, in particular smartphones) on the survey answers and their quality. Indeed, the devices vary at several levels: size of the screen, kind of keyboard, place and conditions (e.g. presence of other persons) in which the respondents use them. In this webinar, we will use evidences from previous literature and from a cross-over experiment that we implemented using the Netquest online access panel in Spain in early 2015 to give insights on which results are really affected by the device used to answer web surveys, and which are not. To get a more general view, we will consider different aspects, like the kind of keyboard and its direction, the survey context and survey experience or the social desirability bias, and different question formats (e.g. open questions, grids, order-by-click).