Table 2.

Definition and calculation of engagement measures.

Engagement measures		Definition
Retentiona
	Completers	Participants who came to the final sessions
	Noncompleters	Participants who did not come to the final sessions (dropout attrition)
	Retention rate	Percentage of completers out of all 55 participants
Fit.healthy.me app usage
	Length of usage	The mean number of days of usage
	Frequency of usage	The mean number of times participants used the app and each feature
	Nonusage attrition	Participants who did not use the app at all in the last month of the study
Fitbit Flex 2 tracker usage
	Length of usage	The mean number of days a valid step count was logged
	A valid day of step count	Having at least ten hours of wear time
	Wear time	Calculated by subtracting nonwear time from 24 hours
	Nonwear time	Defined if no step counts were detected over a period of at least sixty continuous minutes, allowing for 2 min of counts between 0 and 100 [49,50]

^aAdapted from Eysenbach (2005) [43].