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\subsection{Measurement of physical activity}  The Fitbit activity monitor Zip (figure \ref{Fitbit})  contains a tri-axial accelerometer to record time-stamped physical activity data. data, including number of steps and physical activity according to four levels: sedentary, lightly active, fairly active and very active. It can be clipped to various locations on the body. For the study we asked participants to clip the device to the inside of their trouser pocket.  TODO: how often did we measure  - has it been validated now?  - explain how data were wirelessly uploaded