How Many Hours Are Enough? Accelerometer Wear Time May Provide Bias in Daily Activity Estimates

<jats:sec sec-type="background"><jats:title>Background:</jats:title><jats:p>There is little consensus on how many hours of accelerometer wear time is needed to reflect a usual day. This study identifies the bias in daily physical activity (PA) estimates caused by accelerometer wear time.</jats:p></jats:sec><jats:sec sec-type="methods"><jats:title>Methods:</jats:title><jats:p>124 adults (age = 41 ± 11 years; BMI = 27 ± 7 kg·m<jats:sup>-2</jats:sup>) contributed approximately 1,200 days accelerometer wear time. Five 40 day samples were randomly selected with 10, 11, 12, 13, and 14 h·d<jats:sup>-1</jats:sup> of wear time. Four semisimulation data sets (10, 11, 12, 13 h·d<jats:sup>-1</jats:sup>) were created from the reference 14 h·d<jats:sup>-1</jats:sup> data set to assess Absolute Percent Error (APE). Repeated-measures ANOVAs compared min·d<jats:sup>-1</jats:sup> between 10, 11, 12, 13 h·d<jats:sup>-1</jats:sup> and the reference 14 h·d<jats:sup>-1</jats:sup> for inactivity (<100 cts·min<jats:sup>-1</jats:sup>), light (100−1951 cts·min<jats:sup>-1</jats:sup>), moderate (1952−5724 cts·min<jats:sup>-1</jats:sup>), and vigorous (≥5725 cts·min<jats:sup>-1</jats:sup>) PA.</jats:p></jats:sec><jats:sec sec-type="results"><jats:title>Results:</jats:title><jats:p>APE ranged from 5.6%−41.6% (10 h·d<jats:sup>-1</jats:sup> = 28.2%−41.6%; 11 h·d<jats:sup>-1</jats:sup> = 20.3%−36.0%; 12 h·d<jats:sup>-1</jats:sup> = 13.5%−14.3%; 13 h·d<jats:sup>-1</jats:sup> = 5.6%−7.8%). Min·d<jats:sup>-1</jats:sup> differences were observed for inactivity, light, and moderate PA between 10, 11, 12, and 13 h·d<jats:sup>-1</jats:sup> and the reference (<jats:italic>P</jats:italic> < .05).</jats:p></jats:sec><jats:sec sec-type="conclusions"><jats:title>Conclusions:</jats:title><jats:p>This suggests a minimum accelerometer wear time of 13 h·d<jats:sup>-1</jats:sup> is needed to provide a valid measure of daily PA when 14 h·d<jats:sup>-1</jats:sup> is used as a reference.</jats:p></jats:sec>