Sex Differences in the Variability of Physical Activity Measurements Across Multiple Timescales Recorded by a Wearable Device: Observational Retrospective Cohort Study - Kristin J Varner; Lauryn Keeler Bruce; Severine Soltani; Wendy Hartogensis; Stephan Dilchert; Frederick M Hecht; Anoushka Chowdhary; Leena Pandya; Subhasis Dasgupta; Ilkay Altintas; Amarnath Gupta; Ashley E Mason; et al. (Journal of Medical Internet Research)