Page 371 - Contributed Paper Session (CPS) - Volume 4

CPS2292 Roger S. Zoh, PhD et al.
post follow up. The count of steps represents the number of steps taken over
a given period of time and is an indicator of a subject's physical activity levels.
Current guidelines for recommended daily physical activity levels are based on
the duration of time spent in either moderate or vigorous intensity activity
levels and number of steps per day (Matthews, 2012). For example, Tudor et
al. (2004) indicated that activity levels of 12,000 steps/day and 15,000
steps/day for boys and girls, respectively were recommended for maintenance
of healthy body composition for children between the ages of 6-12 years.
While daily energy expenditure is defined as the total number of calories or
energy used by the body to perform daily bodily functions.

Table 1 Descriptive statistics for the study sample at baseline (n=255).
"Other"=Asians/Native Americans, EE= energy expenditure, SD=standard
deviation.

In our application, energy expenditure and step counts were both
collected per minute from the SenseWear Armband® (BodyMedia, Pittsburgh,
PA) among the 374 children enrolled in the study who wore accelerometers
while in school for one week at baseline. The children's body weight, height,
age, and sex were all collected at baseline, while their BMI's were calculated at
the beginning of each semester over the study period. True daily energy
expenditure behaviour, X(t), was considered the latent covariate. The surrogate
measure for $X(t)$ was the energy expenditure taken per hour obtained from
the device, W(t). Step counts measured by the device was treated as the
instrumental variable in this application, M(t). We assume that $cov\{X(t),M(t)\}
\neq 0$ and $cov\{M(t),U(t)\} = 0$. Justification of the use of instrumental
variables is challenging in practice. However, an instrumental variable may be
based on a separate independent measure of X(t). In our application, both M(t)
and W(t) were obtained from the same device. But their measured or
calculated measures were obtained separately. The SenseWear Armband®
obtained the step count based on a 3-axis accelerometer and pattern

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