Page 372 - Contributed Paper Session (CPS) - Volume 4

CPS2292 Roger S. Zoh, PhD et al.
recognition. While the calculation of total energy expenditure was based on
heat flux, skin temperature, galvanic skin response, and anthropometrics (Lee,
2015).
To assess impacts of energy expenditure obtained at baseline on the
difference in BMI values among the enrolled students, we first assumed that
both W and M were discretely observed on a time interval [0, T]. On average,
the students wore the devices for six hours on each school day during the
week it was worn at baseline. Since the accelerometry data were collected per
minute, we combined all the data for the week the device was worn and
averaged all the minute-level data collected within the week to hourly-level
data to reduce any potential noise associated with the data collection. Figure
1 provides the plot of Wi(t) and Mi(t) against time for all subjects included in
the study. The grey lines illustrate the individual trajectories while the blue
solid line is the smoothed mean for the observed energy expenditure and step
counts among all the subjects. Two sets of analyses were performed to
illustrate our developed methods. We first assessed the relationship between
energy expenditure and BMI at baseline. The second analysis involved
investigating the impact of energy expenditure at baseline on changes in BMI
values at 18 months follow up. Due to loss of follow up or missing data, 255
and 156 students contributed to the baseline and the 18-month follow up
analyses, respectively. The average BMI values at baseline was 17.4 kg/m2 (SD
= 2.98) and 17.6 kg/m2 (SD = 3.2) during the spring semester of the second
academic year. The mean step counts per hour at baseline was 13.16 (SD =
11.5) and the mean energy expenditure at baseline was 1.21 kcal/hour $(SD =
0.41), while the average age of the children at baseline was 7.9 years (SD =
0.80). About n=174 (68.24%) were whites, blacks n=34 (13.33%), Hispanics
n=25 (9.8%) and others n=22 (8.63%). See Table 1 for details.
We provide the results from the baseline analyses and the follow up
analyses in Figure 2. Plots of the estimated functional coefficient and the
estimated 95% point-wise confidence intervals are provided in the figure. For
assessments of the impact of energy expenditure on BMI at baseline, the
bootstrap confidence intervals did not contain the zero line completely,
indicating that the functional coefficient was not zero across the whole time
space. Similarly, in determining the impacts of baseline measures of energy
expenditure on the 18-month change in BMI over the study period, the
estimated bootstrap confidence intervals did not contain the zero line
completely. Because the function-valued coefficient was not completely zero
across time, there was some statistical evidence of a relationship between
baseline measures of energy expenditure and BMI values obtained at a future
time, such as 18 months post baseline. Additionally, the relationship observed
depended on both the level of energy expenditure and time.

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