Unit-specific calibration of Actigraph accelerometers in a mechanical setup – Is it worth the effort? The effect on random output variation caused by technical inter-instrument variability in the laboratory and in the field

Abstract

BackgroundPotentially, unit-specific in-vitro calibration of accelerometers could increase field data quality and study power. However, reduced inter-unit variability would only be important if random instrument variability contributes considerably to the total variation in field data. Therefore, the primary aim of this study was to calculate and apply unit-specific calibration factors in multiple accelerometers in order to examine the impact on random output variation caused by inter-instrument variability.MethodsInstrument-specific calibration factors were estimated in 25 MTI- and 53 CSA accelerometers in a mechanical setup using four different settings varying in frequencies and/or amplitudes. Calibration effect was analysed by comparing raw and calibrated data after applying unit-specific calibration factors to data obtained during quality checks in a mechanical setup and to data collected during free living conditions.ResultsCalibration reduced inter-instrument variability considerably in the mechanical setup, both in the MTI instruments (raw SDbetween units = 195 counts*min-1 vs. calibrated SDbetween units = 65 counts*min-1) and in the CSA instruments (raw SDbetween units = 343 counts*min-1 vs. calibrated SDbetween units = 67 counts*min-1). However, the effect of applying the derived calibration to children's and adolescents' free living physical activity data did not alter the coefficient of variation (CV) (children: CVraw = 30.2% vs. CVcalibrated = 30.4%, adolescents: CVraw = 36.3% vs. CVcalibrated = 35.7%). High correlations (r = 0.99 & r = 0.98, respectively) were observed between raw and calibrated field data, and the proportion of the total variation caused by the MTI- and CSA monitor was estimated to be only 1.1% and 4.2%, respectively. Compared to the CSA instruments, a significantly increased (9.95%) mean acceleration response was observed post hoc in the batch of MTI instruments, in which a significantly reduced inter-instrumental reliability was observed over time.ConclusionThe application of unit-specific calibration factors to data collected during free living conditions had no apparent effect on inter-instrument variability. In all probability, the effect of technical calibration was primarily attenuated in the field by other more dominant sources of variation. However, routine technical assessments are still very important for determining the acceleration responses in the batch of instruments being used and, if performed after every field use, for preventing decidedly broken instruments from being returned into the field repeatedly.

Comparison of Four ActiGraph Accelerometers during Walking and Running

“…This was done to minimize the systematic error that could be attributable to the device (15). Following the completion of data collection, the unit was returned to the manufacturer to verify that there were no changes in calibration, thus ensuring consistent calibration of the device during the data collection.…”

The use of individual cut points from treadmill walking to assess free-living moderate to vigorous physical activity in obese subjects by accelerometry: is it useful?

“…The attachment of the accelerometer may be a crucial aspect of the precision of the measurement process because tilting of the accelerometer or small differences in hip-placement are known to influence the accelerometer output [10,28]. We expected that the cut points derived from two accelerometers would be fairly similar and that typical deviations would be under 8.9% [10-12]. Because our data were homoscedastic, such a percentage difference is difficult to evaluate.…”

“…However, because reliability is a premise for validity [8], research exploring measurement variability is important to reveal the different sources of variation (reviews of reliability studies for accelerometer measurement can be found elsewhere [5,9]). For instance, Actigraph instruments (Actigraph, Fort Walton Beach, FL, USA, formerly known as Computer Science and Applications (CSA) and Manufacture Technology Incorporated (MTI) models) are reported to have an inter-instrument coefficient of variation (CV) of under 8.9% [10-12] and an intra-instrument CV of under 4.4% [10,13]. The inter-instrument variation could be addressed through an individual unit calibration.…”

“…The inter-instrument variation could be addressed through an individual unit calibration. However, a study by Moeller et al [12] estimated that such a calibration explained under 4.2% of the variation in field data for Actigraph instruments. Hence, individual instrument calibration may not be worth the effort.…”

Linear Acceleration Measurement Utilizing Inter-Instrument Synchronization: A Comparison between Accelerometers and Motion-Based Tracking Approaches

“…Many authors have developed novel calibration methods (e.g., Krohn, Beigl, Decker, Kochendörfer, Robinson, & Zimmer, 2005). Most manufacturing companies will also perform their own calibration to ensure inter-instrument reliability, but details on this practice are often undisclosed for proprietary reasons (Moeller, Korsholm, Kristensen, Andersen, Wedderkopp, & Froberg, 2008).…”