The past decade has seen an exponential increase in the availability of affordable, consumer-oriented devices for monitoring physical activity. The relatively low prices and attractive design of these products make them appealing to researchers who incorporate physical activity monitoring into their studies, since these features make it possible to utilize the monitors on large samples and improve participants’ compliance with wearing the device, but data on device validity is needed to support using these consumer-oriented activity monitors in a scientific context. One of the most successful companies competing in this area is Fitbit, which offers a line of products that vary in terms of how they are worn (e.g., clipped onto clothing or worn on a wrist) as well as in terms of their features (e.g., rechargeable versus battery-powered; different types of visual displays). In 2015, Fitbit held 27 % of the market share in wearable devices, ahead of Apple at 15 % [1].

The Fitbit Zip is one consumer-oriented device that has several features that make it attractive to researchers. Firstly, it is relatively affordable, (60 U.S. Dollars each), thus making it amenable to use with large samples. Secondly, it syncs wirelessly through Bluetooth, thus making transfer of data very easy. Thirdly, it runs on a replaceable battery that is projected to support continuous function for up to 6 months, meaning that the device does not have to be recharged every few days. Finally, it is small, comes in a variety of colors, and can be worn on the waist, in a pocket, or attached to undergarments; all features that make it more acceptable to users and therefore increase compliance from research participants. All of these features make the Fitbit Zip very appealing to researchers who wish to document physical activity among free-living individuals over longer periods of time.

The Fitbit Zip has been validated within a laboratory situation and has been found to be quite accurate in detecting number of steps when an adult is walking on a treadmill [2, 3], engaging in a variety of activities in a supervised setting [4], or participating in free-living activities [3]. More relevant to field-based studies, a high correlation (r = .91) has been found between daily step counts recorded by the Fitbit and the ActiGraph GT3X (considered a highly accurate research device for monitoring activity) among 47 free-living adults who wore the device for 7 days [5]. Similarly, a study of 21 adults who wore the Fitbit Zip and the ActiGraph for 48 h confirmed the high correlation between the Fitbit Zip and the ActiGraph for step counts and also showed a high correlation (r = .88) between the measures of moderate-to-vigorous physical activity yielded by each device [6]. All of the studies cited above were conducted among healthy adults.

These field-based studies go a long way toward building confidence in the Fitbit Zip as a device that can be expected to collect valid data in a community-based study, but more information is needed about their use among youth and their reliability for collecting longitudinal data. Adolescence is a developmental stage during which activity levels typically decline dramatically [7], and a call to action has been made to develop effective means of arresting this decline [8]. Most research studies include multiple time points of data collection, so it is critical to evaluate the validity of the data collected by these devices at multiple time points. Evidence for the validity and reliability of the Fitbit Zip as a tool that may be used in a supervised laboratory setting can now be considered fairly strong. However, initial forays into documenting the validity of the Fitbit Zip among free-living adults, where data quality can be impacted by undetected device malfunctions, non-wearing of the device, or issues with data loss or transfer, has revealed a number of factors that may limit the utility of this device outside the laboratory (e.g., potential for loss, battery failure, data uploading issues, etc.… [9]). Because youth engage in different patterns of physical activity as compared to adults [10], it is important that activity monitors be validated among youth as well as among adults. Obtaining valid measures of youth physical activity is necessary to evaluate behavior change interventions and investigate the link between physical activity and health among youth [11]. The purpose of this study was to investigate both the feasibility and the validity of using the Fitbit Zip to assess physical activity among free-living adolescents.

The proliferation of consumer-friendly activity monitoring devices has made physical activity research more accessible to investigators, but validation data in the field have been lacking, particularly with respect to using these devices among youth. This study set out to validate the use of the Fitbit Zip as a means of assessing physical activity levels among free-living adolescents. The results provide strong evidence of validity, yet also confirm concerns about potential changes in algorithms over time. Specifically, the Fibit correlated quite strongly with the ActiGraph among all three Cohorts for both estimates of steps per day and also estimates of daily MVPA. However, the Fitbit tended to overestimate steps overall, and overestimated MVPA by a considerable amount in Cohort One. Because this shift over time was so dramatic, we suspected it might be related to a change in the way that Fitbit processes the data, and in fact it emerged that Fibit modified their algorithms in April, 2015 (in between our Cohort 1 and Cohort 2 data collections) to include only activity that persisted for at least 10 min in their computation of “active minutes” (see https://www.help.fitbit.com/articles/en_US/Help_article/1379 for details). This change explains why the correspondence between the estimates of MVPA derived from the two devices was so much better for Cohorts Two and Three.

The Fibit’s overestimation of steps per day in comparison to the ActiGraph is consistent with the findings of Tully et al. [5]. These investigators found that the Fitbit overestimated steps by about 700 steps per day, which is a difference of a similar magnitude to what we found. Because the algorithms used to convert raw Fitbit data into indexes of activity (MVPA and steps) are not publicly available, it must be assumed that the company has chosen a threshold for steps that is lower than that used by the ActiGraph algorithm. There is vigorous debate and no generally-accepted standard for which algorithm is the most valid [18], so the mere fact that a different algorithm is used is not grounds for rejecting the Fitbit. It is important, however, to be cognizant that activity metrics obtained from the Fitbit should probably not be treated as equivalent to activity metrics obtained from the ActiGraph. However, the MVPA data obtained from Cohorts Two and Three in this study showed very close correspondence between the Fitbit and the ActiGraph (i.e., very high correlations and no statistical difference between the means), suggesting that the Fitbit might in fact offer a less expensive alternative to the ActiGraph.

It has been argued that consumer-oriented devices such as the Fitbit Zip may offer a valuable adjunct to behavior-change interventions by providing accessible feedback regarding activity levels, but should not replace research-grade devices in collecting data to be used in scientific analyses [19]. A strong objection to using consumer-oriented devices to collect research data is that the companies guard their algorithms closely, which leaves scientists in the dark as to whether the algorithms have been changed over time and how the algorithms relate to those of known devices, such as the ActiGraph. The present study gives credence to this concern, and should raise alarms about the possibility that a device company might alter their algorithms in the middle of a longitudinal research study.

Clinicians also are eager to have access to an accurate and reliable device to track activity among their patients, and lab-based studies have been presented as evidence for using the Fitbit in a clinical setting [20]. However, as this study demonstrates, accuracy in a controlled laboratory setting may not translate to validity and reliability over time. Our results suggest that both clinical and research utilization of consumer-oriented devices to measure and assess physical activity should proceed cautiously and that additional evidence of field-based performance is critical.

Another point that should be made is that these adolescents were wearing an ActiGraph on a belt throughout the week of data collection. The Fitbit was affixed to the ActiGraph belt. Consequently, there was no need for the study participant to keep track of the Fitbit. It has been our general experience working with this device, which clips onto a pocket or waistband, that individuals do not infrequently forget either to remove the device from their clothing at the end of the day (at times resulting in the device making a journey through a washing machine) or to attach the device to their clothing in the morning, resulting in data loss. Because the ActiGraph belt is more difficult to overlook, participants generally have an easier time remembering to remove it and replace it as instructed. Recently, Fitbit has released a line of wrist-worn devices that employ similar technology for slightly greater expense. Because these wrist-worn devices are less intrusive and can be worn 24 h per day, they may solve this issue of wearability. Studies are beginning to emerge validating the wrist-worn monitors [4], and it is likely that if they prove valid and reliable they will be seen as preferable to the waist-worn devices.

Practically speaking, this study suggests that the Fitbit Zip is a reliable assessment tool among adolescents when worn for at least 4 days and when used within a period of time during which Fitbit does not modify their algorithms. Because it is far more affordable than the ActiGraph, the Fitbit Zip may therefore be a feasible tool for assessing activity levels across large groups of youth, perhaps within a school setting, as a means of tracking group changes in activity over time.

Several studies have previously demonstrated the validity of the Fitbit in a controlled laboratory setting. This study takes the analysis out into the field and evaluates the device among free-living adolescents. The results offer confirmation of a high correlation between Fitbit and ActiGraph data for both MVPA and steps per day, although steps per day tend to be overestimated by the Fitbit in comparison to the ActiGraph. It should also be noted that a large part of the appeal of the Fitbit is the potential for using it over longer periods of time than a 7-day assessment. This study does not provide information about the utility of the Fitbit for collecting data over periods longer than 1 week.