Participants were recruited via respondent-driven sampling on the basis of reported friends; the study further used this friendship relation to automatically detect (using on-phone Bluetooth technology) cases in which a participant was in close proximity to the friend. To monitor message exposure, the system made use of communicational monitoring, on-device self-reporting, and sensor data. To capture exposure to online messaging, Ethica Health monitored participant browsing to webpages containing certain tobacco-related keywords, as well as other randomly selected page visits. To capture understanding of placement of and exposure to other messaging, participants were asked to indicate encounters with and photograph messaging which they encountered, using a study-specific button in the Ethica Health-provided interface. The system then enquired about the participant’s reaction to the images, and the images submitted via such mechanisms were automatically geotagged with the location where they were taken, as is typical with all questions and other data collected by Ethica Health. Questionnaires were triggered by proximity to personal contacts as well as occurrence of browsing, as well as at random times during the day (such as to sense tobacco use by the participant and others).