title: The relationship between mobile phone location sensor data and depressive symptom severity.
abstract: BACKGROUND
  Smartphones offer the hope that depression can be detected using passively collected data from the phone sensors. The aim of this study was to replicate and extend previous work using geographic location (GPS) sensors to identify depressive symptom severity.
  
  
  METHODS
  We used a dataset collected from 48 college students over a 10-week period, which included GPS phone sensor data and the Patient Health Questionnaire 9-item (PHQ-9) to evaluate depressive symptom severity at baseline and end-of-study. GPS features were calculated over the entire study, for weekdays and weekends, and in 2-week blocks.
  
  
  RESULTS
  The results of this study replicated our previous findings that a number of GPS features, including location variance, entropy, and circadian movement, were significantly correlated with PHQ-9 scores (r's ranging from -0.43 to -0.46,p-values <  .05). We also found that these relationships were stronger when GPS features were calculated from weekend, compared to weekday, data. Although the correlation between baseline PHQ-9 scores with 2-week GPS features diminished as we moved further from baseline, correlations with the end-of-study scores remained significant regardless of the time point used to calculate the features.
  
  
  CONCLUSIONS
  Our findings were consistent with past research demonstrating that GPS features may be an important and reliable predictor of depressive symptom severity. The varying strength of these relationships on weekends and weekdays suggests the role of weekend/weekday as a moderating variable. The finding that GPS features predict depressive symptom severity up to 10 weeks prior to assessment suggests that GPS features may have the potential as early warning signals of depression.
authors.affiliation: Department of Preventive Medicine, Northwestern University, Chicago, IL, United States; Rehabilitation Institute of Chicago, Department of Physical Medicine and Rehabilitation, Northwestern University, Chicago, IL, United States.
authors.forename: Sohrab
authors.initials: S
authors.lastname: Saeb
authors.affiliation: Department of Preventive Medicine, Northwestern University , Chicago , IL , United States.
authors.forename: Emily G
authors.initials: EG
authors.lastname: Lattie
authors.affiliation: Department of Preventive Medicine, Northwestern University , Chicago , IL , United States.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Schueller
authors.affiliation: Rehabilitation Institute of Chicago, Department of Physical Medicine and Rehabilitation, Northwestern University , Chicago , IL , United States.
authors.forename: Konrad P
authors.initials: KP
authors.lastname: Kording
authors.affiliation: Department of Preventive Medicine, Northwestern University , Chicago , IL , United States.
authors.forename: David C
authors.initials: DC
authors.lastname: Mohr
keywords: Depression; Depressive symptoms; Geographic locations; Mobile phone; Students