How many days are needed? Measurement reliability of wearable device data to assess physical activity

Sep 17, 2026

Wearable devices are widely used to estimate participants’ habitual physical activity by averaging across several valid observation days. Competing study demands require knowing how many valid days are sufficient to achieve reliable measurement of long-term activity averages. This article evaluates long-term test-retest reliability of aggregate activity measures using resampling methods in 79 participants who wore a Fitbit Flex for about one year, and compares results with conventional short-window reliability approaches.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages14

Reading Time35 min

AccessFree to download

Abstract

Introduction/Purpose

Methods

Results

Conclusion

Introduction

PLOS ONE OPEN ACCESS Citation: Hilden P, Schwartz JE, Pascual C, Diaz KM, Goldsmith J (2023) How many days are needed? Measurement reliability of wearable device data to assess physical activity. PLoS ONE 18(2): e0282162 . [https://doi.org/10.1371/journal](https://doi.org/10.1371/journal). pone.0282162 Editor: Ender Senel, Mugla Sitki Kocman University: Mugla Sitki Kocman Universitesi, TURKEY Received: October 31, 2022 Accepted: February 9, 2023 Published: February 24, 2023 Copyright: © 2023 Hilden et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All study data are available at [https://osf.io/kmszn/](https://osf.io/kmszn/). Funding: Collection of the data analyzed in this study was supported by National Heart Lung and Blood Institute grant R01HL115941. The fundershad no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist. RESEARCH ARTICLE How many days are needed? Measurement reliability of wearable device data to assess physical activity Patrick Hilden1, Joseph E. Schwartz2,3, Christian Pascual4, Keith M. Diaz2, Jeff Goldsmith1 * 1 Department of Biostatistics, Mailman School of Public Health, Columbia University, New York, New York, United States of America, 2 Center for Behavioral Cardiovascular Health, Department of Medicine, Columbia University Irving Medical Center, New York, NY, United States of America, 3 Department of Psychiatry and Behavioral Sciences, Renaissance School of Medicine, Stony Brook University, Stony Brook, NY, United States of America, 4 Department of Biostatistics, Herbert Wertheim School of Public Health, University of California, San Diego, San Diego, CA, United States of America * [jeff.goldsmith@columbia.edu](jeff.goldsmith@columbia.edu) Abstract Introduction/Purpose Physical activity studies often utilize wearable devices to measure participants’ habitual activity levels by averaging values across several valid observation days. These studies face competing demands–available resources and the burden to study participants must be balanced with the goal to obtain reliable measurements of a person’s longer-term average. Information about the number of valid observation days required to reliably measure targeted metrics of habitual activity is required to inform study design. Methods To date, the number of days required to achieve a desired level of aggregate long-term reliability (typically 0 .80) has often been estimated by applying the Spearman-Brown Prophecy formula to short-term test-retest reliability data from studies with single, relatively brief observation windows. Our work, in contrast, utilizes a resampling-based approach to quantify the long-term test-retest reliability of aggregate measures of activity in a cohort of 79 participants who were asked to wear a FitBit Flex every day for approximately one year. Results The conventional approach can produce reliability estimates that substantially overestimate the actual test-retest reliability. Six or more valid days of observation for each participant appear necessary to obtain 0.80 reliability for the average amount of time spent in light physical activity; 8 and 10 valid days are needed for sedentary time and moderate/vigorous activity respectively. Conclusion Protocols that result in 7–10 valid observation days for each participant may be needed to obtain reliable measurements of key physical activity metrics. Introduction Studies that use wearable devices often produce daily summary metrics like time spent in sedentary, light (LPA), and moderate to vigorous physical activity (MVPA), in order to quantify daily physical activity for

FAQs

Why is the number of valid observation days important in wearable-based physical activity studies?

Researchers often average multiple valid days to estimate habitual activity, but study resources and participant burden must be balanced. Knowing how many days are sufficient helps ensure reliable measurement of longer-term averages.

How did the study estimate long-term measurement reliability?

It used a resampling-based approach to quantify long-term test-retest reliability of aggregate activity measures, using data from 79 participants wearing a Fitbit Flex for about one year.

What number of valid days is suggested for reliable estimates of key activity metrics?

Six or more valid days were needed for reliable light physical activity time (about 0.80 reliability). Sedentary time required 8 valid days, and moderate/vigorous activity required 10 valid days; protocols with roughly 7–10 valid observation days may be needed overall.

Sep 17, 2026

Uploaded by Taylor Morgan