Hi, everybody
I am trying to use the regression trees to map some soil attributes of
tropical hillslope areas with rpart package.
The regression trees shows good results (in r2 evaluation = 0.28 - it is
not so bad for soil attributes mapping comparing wiht others models like
"lm" or geostats),
but I don´t understand the output of "xpred.rpart" command.
This is the result:
###

> printcp(silt3.fit)
Regression tree:
rpart(formula = silt.3 ~ MDE15FILLE + MID.SLOPE_, data = dataset,
    method = "anova")

Variables actually used in tree construction:
[1] MDE15FILLE MID.SLOPE_

Root node error: 263226/133 = 1979.1

n= 133

        CP nsplit rel error  xerror    xstd
1 0.095908      0   1.00000 1.02232 0.15190
2 0.040496      1   0.90409 0.99901 0.14677
3 0.028707      2   0.86360 1.06906 0.15292
4 0.028383      4   0.80618 1.03300 0.15267
5 0.023571      5   0.77780 1.02786 0.14994
6 0.018586      6   0.75423 1.03126 0.13944
7 0.012423      7   0.73564 1.05174 0.14205
8 0.010000      8   0.72322 1.09176 0.15294


> xpred.rpart(silt3.fit)[1:10,]   0.54795416 0.06232064 0.03409537 0.02854415 0.02586509 0.02093086 0.01519568 0.01114606
1    148.0317   138.8494   138.8494   147.2283   159.0290   159.0290
159.0290   174.1000
2    149.7717   141.1080   141.1080   141.1080   141.1080   151.8475
178.4000   178.4000
14   146.0733   168.6925   152.7095   152.7095   152.7095   152.7095
152.7095   160.8357
15   148.3492   140.4056   140.4056   140.6931   140.6931   140.6931
140.6931   140.6931
23   148.3492   171.1548   153.1118   153.1118   153.1118   153.1118
153.1118   153.1118
39   148.5101   170.6806   149.9600   149.9600   149.9600   149.9600
149.9600   149.9600
52   148.5101   170.6806   149.9600   149.9600   149.9600   149.9600
149.9600   149.9600
53   148.3492   140.4056   140.4056   140.6931   140.6931   140.6931
140.6931   140.6931
56   148.3492   140.4056   140.4056   152.8780   158.6294   158.6294
158.6294   158.6294
57   146.0733   168.6925   152.7095   152.7095   152.7095   152.7095
152.7095   136.4571

####
I would like to know if each column is the cross validation for each
variable? And what type of cross validation is use? Leave-one-out? the xval
is not set.
The CP - complexity parameter, for each column, is different from the
"printcp" command, why ?
how can I calculate the R2 of the cross validation? or, is the "xstd"
column of "printcp" comand teh cross validation?
thanks for any help

-- 
________________________
Waldir de Carvalho Junior
Pedologia
Pesquisador
Embrapa Solos

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