The
cross-sectional nature of this data is a relative weakness.
While SEM is able to test whether directional, or causal,
relationships are possible based on a particular set of
data, a definitive test of causality would require a large,
longitudinal dataset in which, for instance, the impact of
sleep problems at one time point could be assessed on
physical functioning, emotional functioning, and/or
functional disability at later time points. Future longitudinal
work also may allow for examining the impact of
interventions to target particular components of the
model on functional disability. In this manner, the most
appropriate and powerful target(s) for intervention
efforts may be identified.