The accelerometry data were cleaned to remove nonwear time, defined as $60 minutes of zero counts. People with COPD are very sedentary, making it possible to label data as non wear time when it was actually prolonged inactivity. A second accelerometer was worn on the wrist of the dominant hand, and for purposes of this report, the wrist data were used to validate wear time for the waist-mounted accelerometers. Subjects maintained a daily activity log that was used to facilitate interpretation of accelerometry data. We assumed that waist and wrist accelerometers were both worn together, and when the duration of zero counts was $60 minutes for data collected from the waist accelerometer, we checked the wrist data to determine if the wrist was active. If data from the wrist accelerometer and data from the activity log indicated that the subject was still being monitored, then the waist data were treated as valid and not removed as non wear time.