Validity and reliability of the Self-administered Psycho-TherApy-SystemS (SELFPASS) item pool for the daily mood tracking of depressive symptoms: cross-sectional web-based survey - Mayer, Gwendolyn; Hummel, Svenja; Gronewold, Nadine; Oetjen, Neele; Hilbel, Thomas; Schultz, Jobst-Hendrik (JMIR mental health)