Together with Daniel Gilbert, a Harvard professor of psychology, Killingsworth used the app to regularly check in with a total of 15,000 people who were chosen at random. He spoke to them regarding their activities, feelings, and thoughts at various times of the day. The app did this by sending surveys to the subjects' phones. Participants were required to fill out the forms about what they were doing, whether they were thinking about what they were doing, and how happy they felt.-