st: plotting confidence bands around marginal means from mixed model

[Eric Jacobson <eric_jacobson@hms.harvard.edu>]

Dear List,
	I analyzed longitudinal data on outcomes from a randomized controlled clinical trial by building a linear mixed model using using xtmixed in Stata 12,

xtmixed vaspain tx_group days tx_group##c.days || studycode: days, reml covar(un)

vaspain is the continuous outcome.
tx_group is binary 1/0
days is continuous time variable.
studycode are integers 1-46 that identify each individual subject in the trial.

	So this model has main effects for tx_group, days, and the interaction of tx_groupXdays; it also has random intercept and slope for each studycode

	I found a significant tx_groupXdays effect, i.e. a significant difference between the two tx_group in the slopes for decline of vaspain over days.

	I have plotted the regression lines for the marginal effect of vaspain vs. days, by each tx_group; i.e. one line for group 1, another line for group 2.  That seems fairly straight forward.  
	Now I would like to put 95% confidence intervals around each line, i.e. the CIs that reflect the marginal variance in the mixed model, not the CI from an ordinary linear regression.
 	I have been unable to find out how to do the latter.  It seems like it might be possible using either predict and two-way, or margin and marginplot; but lots of searching for directions or an example has turned up nothing.

	Does anyone know how to produce such a plot from an xtmixed model? Is it possible to do this in STATA 12?
	
Thanks for any enlightenment, Eric

Eric Jacobson
Department of Global Health & Social Medicine
Harvard Medical School





*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/