For reasons of refinement, we analyzed in detail the methods that were used in the 17 survey studies. The majority of survey studies (15 publications) are based on (online) questionnaires. Five publications used more than one method: Conti et al. (2012) and Okazaki and Taylor (2008) combined questionnaires and interviews. Truong and Simmons (2010) conducted a qualitative study based on interviews. Vatanparast and Ali Hasan (2010) is the only publication in the sample that used a panel, and Tussyadiah (2012) is the only one using focus groups in the research design.