  We develop methods for analyzing discrete multivariate longitudinal data and
apply them to functional disability data on the U.S. elderly population from
the National Long Term Care Survey (NLTCS), 1982-2004. Our models build on a
Mixed Membership framework, in which individuals are allowed multiple
membership on a set of extreme profiles characterized by time-dependent
trajectories of progression into disability. We also develop an extension that
allows us to incorporate birth-cohort effects, in order to assess
inter-generational changes. Applying these methods, we find that most
individuals follow trajectories that imply a late onset of disability, and that
younger cohorts tend to develop disabilities at a later stage in life compared
to their elders.
