[R] Method for selection bias with multinomial treatment

[ronggui]

I have to the treatment effect base on the observational data.And the
treatment variable is multinomial rather than  binary.Because the
treatment assignment is not random,so the selection-bias exists.Under
this condition,what's the best way to estimate the treatment effect?

I know that if the treatment is binary,I can use propensity score
matching using MatchIt package.But what about multinomial?

Maybe one way is the method  proposed by Imbens,2000,The role of the
propensity score in estimating dose-response
functions,Biometrika,87:706-710.But I can't find any R function to
do the task.So I hope the lister can give me some suggestions.

Thank you in advance!

-- 
»ÆÈÙ¹ó
Department of Sociology
Fudan University