[R] predict rpart: new data has new level

[Chua Siang Li]

   Hi.  I uses rpart to build a regression tree.  Y is continuous. Now, I try
   to predict on a new set of data.  In the new set of data, one of my x (call
   Incoterm, a factor) has a new level.
   I  wonder  why the  error  below appears as the guide says "For factor
   predictors, if an observation contains a level not used to grow the tree, it
   is left at the deepest possible node and frame$yval at the node is the
   prediction. "
   Many thanks.
   > mod <- rpart(y~., data=data.frame(y=y,x=x), method="anova",
   +                       cp=0.05, minsplit=100, minbucket=50, maxdepth=5)
   > predictLost  <-  predict(mod,  newdata=data.frame(y=yLost, x=xLost),
   type="vector")
   Error  in  model.frame.default(Terms, newdata, na.action = act, xlev =
   attr(object,  :
     factor 'x.Incoterm' has new level(s) MTD
   ----
   Chua Siang Li
   Consultant - Operations Research
   Acceval Pte Ltd
   Tel: 6297 8740
   Email: siang.li.chua at acceval-intl.com
   Website: www.acceval-intl.com
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