Is a Step a Step? Criterion-related validity of Self-Reported "Fitbit" step counts in Australian adults.

Abstract: In recent years, the “Fitbit” activity monitor has been used in numerous studies on physical activity and to promote lifestyle changes by increasing daily walking. In addition, the Fitbit is frequently used by people in free-living situations to monitor daily step counts. To date, limited studies have examined the validity of these devices and no criterion-validity studies have been performed. This pilot study examined the criterion validity of self-reported Fitbit step counts from a subset of participants in the Global Corporate Challenge. Adult participants (N=18) at an Australian university wore an Actigraph GT3X+ accelerometer for five or more workdays concurrently with wearing a Fitbit. Participants entered their daily Fitbit step counts on a web-based platform. On average participants achieved 10956(3205) and 12684(3826) daily steps, as measured by accelerometer and Fitbit, respectively. Although this difference was significant (t=6.7, p<0.001), the two measures were significantly correlated (r=0.85, p<0.000). Bland-Altman plots demonstrated that only 5% of points were outside the limits of agreement and there was no differential effect on readings across the range of daily steps. However, there was a consistent overestimation in self-reported Fitbit steps. The Fitbit device produced reliable step counts and seems to be a valid device for monitoring daily steps. A consistent overestimation was observed, but whether this was inaccuracies in the device itself, or issues relating to the self-reporting of steps, is not known.