Variability in the measurement of uptime in children: a preliminary study

Objective: To examine variability in 'uptime' ( the amount of time spent in the upright position). Design: An observational study. Setting: The community (i.e., homes and schools in the Melbourne metropolitan area). Subjects: Normal children between 8 and 15 years of age ( 18 girls and 8 boys). Equipment: An 'uptimer', a lightweight, battery-operated remote monitor, was used to record uptime. Procedure: Each child in the study wore the uptimer continuously over a 24-hour period, which included a typical day at school, on four separate occasions, one week apart. Results: The variability in repeated measures of uptime in the same child ( standard deviation 0.8 hours in 24 hours) was lower than the variability between the children ( standard deviation 1.1 hours in 24 hours). We estimate that a single uptime measurement has reliability given by an ICC of 0.65 (95% confidence interval 0.49 - 0.82), which is calculated by dividing the between-child variability by the total variability (within-child and between-child variability). There were no systematic differences in mean uptime between occasions of measurement. Conclusion: As an estimate of the time that a child spends in the upright position, a single 24-hour period of measurement of uptime may vary considerably from one occasion to the next. However, this within-child variability is less than the variability between children. In large-scale field studies a one-off measurement of uptime may be acceptable, but in small rehabilitation studies uptime should be measured on at least three occasions.