[R] partykit ctree: minbucket and case weights

[Amber Dawn Nolder]

   Hello,
   I am an R novice, and I am using the "partykit" package to create
   regression trees. I used the following to generate the trees:
   ctree(y~x1+x2+x3+x4,data=my_data,control=ctree_control(testtype =
   "Bonferroni", mincriterion = 0.90, minsplit = 12, minbucket = 4,
   majority = TRUE)
   I thought that "minbucket" set the minimum value for the sum of weights
   in each terminal node, and that each case weight is 1, unless otherwise
   specified. In which case, the sum of case weights in a node should equal the
   number of cases (n) in that node. However, I  sometimes obtain a tree with
   a terminal node that contains fewer than 4 cases.
   My data set has a total of 36 cases. The dependent and all independent
   variables are continuous data. Variables x1 and x2 contain missing (NA)
   values.
   Could someone please explain why I am getting these results?
   Am I  mistaken about the value of case weights or about the use of minbucket
   to restrict the size of a terminal node?
   This is an example of the output:
   Model formula:
   y ~ x1 + x2 + x3 + x4
   Fitted party:
   [1] root
   |   [2] x4 <= 30: 0.927 (n = 17, err = 1.1)
   |   [3] x4 > 30
   |   |   [4] x2 <= 43: 0.472 (n = 8, err = 0.4)
   |   |   [5] x2 > 43
   |   |   |   [6] x3 <= 0.4: 0.282 (n = 3, err = 0.0)
   |   |   |   [7] x3 > 0.4: 0.020 (n = 8, err = 0.0)
   Number of inner nodes:    3
   Number of terminal nodes: 4
   Many thanks!
   Amber Nolder
   Graduate Student
   Indiana University of Pennsylvania