The question is about the direction vector in rpart.
There are (at least) two preferred ways to lay out a tree, wrt the question of
which obs are sent left and which right.
1. Send the smaller y values to the left. In the final tree, there will be
a graphical ordering with smaller y's to the left and larger ones to the
right.
One has a "left bad, right good" orientation when traversing the tree.
I find
that medical researchers often like this.
2. Send observations with x < cutpoint to the left. Setting all elements
of
the direction vector to -1 will give this behavior.
I happen to slightly prefer option 1, which of course means that it became
the default behavior in rpart. (For a categorical y with many levels, however,
rpart orders on the percent of observations in category 1, which may not be
particularly useful.)
Terry Therneau
