  Health exams determine a patient's health status by comparing the patient's
measurement with a population reference range, a 95% interval derived from a
homogeneous reference population. Similarly, most of the established relation
among health problems are assumed to hold for the entire population. We use
data from the 2009 - 2010 National Health and Nutrition Examination Survey
(NHANES) on four major health problems in the U.S. and apply a joint mean and
covariance model to study how the reference ranges and associations of those
health outcomes could vary among subpopulations. We discuss guidelines for
model selection and evaluation, using standard criteria such as AIC in
conjunction with posterior predictive checks. The results from the proposed
model can help identify subpopulations in which more data need to be collected
to refine the reference range and to study the specific associations among
those health problems.
