Reproducibility of objectively measured physical activity: Reconsideration needed

ABSTRACT

Reliability of accelerometer-determined physical activity (PA), and thus the required length of a monitoring period, appears to depend on the analytic approach used for its calculation. We compared reliability of objectively measured PA using different resolution of data in a sample of 221 Norwegian 2–6-year-old children providing 2–3 valid 14-day periods of accelerometer monitoring (ActiGraph GT3X+) during September–October, January–February, and May–June 2015–2016. Reliability (intra-class correlation [ICC]) was measured for 1–14 days of monitoring across the measurement periods using linear mixed effect modelling. These results were compared to reliability estimated using different resolution of data using the Spearman–Brown formula. The measured reliability improved only marginally with increased monitoring length and levelled off after 5–6 days. Estimated reliability differed substantially when derived from different resolution of data: 3.9–5.4, 6.7–9.2, 13.4–26.7 and 26.3–87.7 days of monitoring was required to achieve an ICC = 0.80 using an hour-by-hour, a day-by-day, a week-by-week and a period-by-period approach, respectively. Reliability could not be correctly estimated from any single resolution of data. We conclude that reconsideration is needed with regard to how reproducibility of objectively measured PA is analysed and interpreted.

KEYWORDS:

• Test–retest
• reliability
• intra-class correlation
• measurement error
• accelerometry

Acknowledgments

We thank all children, parents and staff at the participating preschools for their excellent cooperation during the data collection. We also thank the Masters students Katrin Oppheim, Janne Øvstetun, Oddbjørn Holmøy and Edvard Veivåg at the Western Norway University of Applied Sciences (formerly Sogn og Fjordane University College) for their assistance during the data collection.

Disclosure Statement

The authors declare that they have no competing interests.

Data Availability

The datasets used in the present study are available from the corresponding author on reasonable request.

Supplementary Materials

Supplemental data for this article can be accessed here.

Additional information

Funding

The study was funded by the County Municipality of Sogn og Fjordane and the Western Norway University of Applied Sciences (formerly Sogn og Fjordane University College). None of the funding agencies had any role in the study design, data collection, analysing or interpreting data, or in writing the manuscripts.