Led by Dr. David C. Mohr, director of Northwestern’s Center for Behavioral Intervention Technologies, the study tracked behaviors that previous studies have linked with depression, including hours spent in certain locations (e.g. work and home) and time spent interacting with the phone. Exactly 40 individuals were chosen from Craigslist to participate in the study; 28 ended up with data sufficient enough to analyze after two weeks.

Each participant downloaded a program the researchers coined the “Purple Robot,” a sensor data acquisition app that relies on things like GPS to gather data about behavior. While other studies have analyzed the daily social interactions of depressed people through phone sensor data, this was the first to explore the phenomenon using an individual’s geographic location.

“The significance of this is we can detect if a person has depressive symptoms and the severity of those symptoms without asking them any questions,” says Mohr. Since phones, according to Mohr, fit into the “fabric of people’s lives,” they could make for a simpler way to measure mental health, without the need for special devices or superfluous paperwork. “We now have an objective measure of behavior related to depression. And we’re detecting it passively. Phones can provide data unobtrusively and with no effort on the part of the user.”