title: Measuring steps with the Fitbit activity tracker: an inter-device reliability study.
abstract: Activity trackers like Fitbit are used for self-tracking of physical activity by an increasing number of individuals. Comparing physical activity scores with peers can contribute to the desired behavioural change. However, for meaningful social comparison a high inter-device reliability is paramount. This study aimed to determine the inter-device reliability of Fitbit activity trackers in measuring steps. Ten activity trackers (Fitbit Ultra) were worn by a single person (male, 46 years) during eight consecutive days. Inter-device reliability was assessed on three different levels of aggregation (minutes, hours, days) with various methods, including intra-class correlation coefficient (ICC), Bland-Altman plots, limits of agreement (LOA) and Mixed Model Analysis. Results showed that the inter-device reliability of the Fitbit in measuring steps is good at all levels of aggregation (minutes, hours, days), but especially when steps were measured per day. This implies that individuals can reliably compare their daily physical activity scores with peers.
authors.affiliation: Hanze University of Applied Sciences, Quantified Self Institute , Groningen , The Netherlands .
authors.forename: Manon L
authors.initials: ML
authors.lastname: Dontje
authors.forename: Martijn
authors.initials: M
authors.lastname: de Groot
authors.forename: Remko R
authors.initials: RR
authors.lastname: Lengton
authors.forename: Cees P
authors.initials: CP
authors.lastname: van der Schans
authors.forename: Wim P
authors.initials: WP
authors.lastname: Krijnen
mesh_terms: D056044:Actigraphy / Q000295:instrumentation*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006801:Humans; D008297:Male; D008875:Middle Aged; D018670:Monitoring, Ambulatory / Q000295:instrumentation*; D009043:Motor Activity / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: Accelerometry; Fitbit; objective measurement; physical activity; reliability