Smartphone apps are increasingly being used for population-based survey research. Recruiting people to sign up for an app-based survey is, however, less straightforward compared to traditional surveys, which risks inflating nonresponse as well as the potential for nonresponse bias. By means of an experiment with over 44,000 recently registered job seekers, we present causal evidence on the effects of using different contact modes (email, postal letter, or preannouncement letter and email) on participation rates in an app-based panel survey. Further, using detailed administrative register data, we investigate whether contact modes differentially affect nonresponse bias. We also examine whether the mode of making contact has a lasting effect on panel participation rates and participation rates in momentary assessments collected using the experience sampling method (ESM). Overall, the preannouncement letter and email invitation strategy maximizes participation compared to stand-alone letters and emails, which do not differ significantly in terms of participation rates. Stand-alone letters and the preannouncement approach perform better than emails when it comes to panel participation and submitted ESM episodes.
