st: Multiple imputation: conditional option using categorical variables

["Hogerbrugge, M.J.A. (Martijn)" <M.J.A.Hogerbrugge@uu.nl>]

Dear Stata users,

Does anybody know how to use the "conditional"-option for categorical variables in multiple imputation using ICE? 

The help file only provides an example using a dichotomous conditional variable ("female"). In that case, imputation on a particular variable is only performed if the condition is met (for instance "female == 1"). If the condition is false, a pre-determined value is imputed. To work correctly, the cases for which the condition is false need to have only one value on the variable that needs to be imputed. If more than one value is observed, ICE returns an error.

In some instances, however, it is perfectly sensible to have multiple values in the variable to be imputed (for the observations for which the condition is false). Let me provide a quick example using the variable marital status:
- the categories for marital status are [1] "married", [2] "divorced", and [3] "widowed"
- marital status is observed at multiple time points
- once a person is divorced or widowed, he or she remains so for the remainder of the observation points
- the ICE command will look like this:

.conditional(marstat_time2: if inlist(marstat_time1,1,.))
.conditional(marstat_time3: if inlist(marstat_time2,1,.))
.conditional(marstat_time4: if inlist(marstat_time3,1,.))

and so on.

Thus, marital status at a later time point is not imputed if the case was either divorced or widowed at the previous time point (i.e., not married [1] or missing [.]).

Unfortunately, ICE returns an error for the reason described earlier. Is there some way to be able to do this kind of conditional imputation nonetheless? I tried looking for previous posts on a similar issue, and found one (http://www.stata.com/statalist/archive/2010-11/msg00708.html), but this post seems to be never been answered.

BTW, I also looked into Stata's own imputation command (MI impute), but the conditional option is implimented in a similar way, and switching commands seems not to be a way to overcome this problem.  

Looking forward to your responses and help.
~Martijn Hogerbrugge 
*
*   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/