In the the last decade, we have noticed a significant increase in the use of web surveys, as well as the usage of new mobile devices. With the rapidly growing usage of mobile devices and development of new mobile technologies, the share of respondents who use smart phones or tablet computers for completing online questionnaire, is also rising. Because of that, data quality that is collected with these new devices is put under question. In this thesis we analyze data that was collected using a web survey of members of an online social forum called "Med.over.net". We focus on differences in quality of answers between respondents who completed the questionnaire with different devices. We compare different data quality indicators, using survey answers as well as paradata. We found differences in three out of five data quality indicators. These are: demographic differences, the share of break-off rates and item-non response level. However, there were no significant difference between smartphone and other users as regards open-ended questions and total time for completing the online questionnaire.