[R] Missing value in Rpart

[Shuguang Chen]

Hi, all

Our understanding of how classification trees in Rpart treat missing is
that if the variable is ordinal(continous), Rpart, by default, imputes a
value for missing.  How do we do the classification tree and tell Rpart not
to impute.  That is, what command is used to turn off the imputation.

Also, if we do get true missing, how does classification tree analysis in
Rpart treat missing when the variable is ordinal (after we tell Rpart not
to impute)?  I believe that when a variable is categorical, missing is
treated as just another category.  But if the variable is ordinal, for
example, if the question is, how satsified are you with such and such, and
the response goes on a 1-5 scale, is the missing treated as another 'value'
on the scale, as a categorical response independent of the 1-5 scale, or as
not part of the response at all, e.g., missing?

Thank you,

Shuguang Chen

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