mob {party} | R Documentation

## Model-based Recursive Partitioning

### Description

MOB is an algorithm for model-based recursive partitioning yielding a tree with fitted models associated with each terminal node.

### Usage

    mob(formula, weights, data = list(), model = glinearModel,
      control = mob_control(), ...)

    ## S3 method for class 'mob':
    predict(object, newdata = NULL, type = c("response", "node"), ...)
    ## S3 method for class 'mob':
    summary(object, node = NULL, ...)
    ## S3 method for class 'mob':
    coef(object, node = NULL, ...)
    ## S3 method for class 'mob':
    sctest(x, node = NULL, ...)

### Arguments

`formula` | A symbolic description of the model to be fit. This should be of type `y ~ x1 + ... + xk | z1 + ... + zl` where the variables before the `|` are passed to the `model` and the variables after the `|` are used for partitioning.
`weights` | An optional vector of weights to be used in the fitting process. Only non-negative integer valued weights are allowed (default = 1).
`data` | A data frame containing the variables in the model.
`model` | A model of class `StatModel-class`. See details for requirements.
`control` | A list with control parameters as returned by `mob_control`.
`...` | Additional arguments passed to the `fit` call for the `model`.
`object, x` | A fitted `mob` object.
`newdata` | A data frame with new inputs, by default the learning data is used.
`type` | A character string specifying whether the response should be predicted (inherited from the `predict` method for the `model`) or the ID of the associated terminal node.
`node` | A vector of node IDs for which the corresponding method should be applied.

### Details

Model-based partitioning fits a model tree using the following algorithm:

1. `fit` a `model` (default: a generalized linear model `glinearModel` with formula `y ~ x1 + ... + xk` for the observations in the current node.
2. Assess the stability of the model parameters with respect to each of the partitioning variables `z1`, ..., `zl`. If there is some overall instability, choose the variable `z` associated with the smallest p value for partitioning, otherwise stop. For performing the parameter instability fluctuation test, a `estfun` method and a `weights` method is needed.
3. Search for the locally optimal split in `z` by minimizing the objective function of the `model`. Typically, this will be something like `deviance` or the negative `logLik` and can be specified in `mob_control`.
4. Re-fit the `model` in both children, using `reweight` and repeat from step 2.

More details on the conceptual design of the algorithm can be found in Zeileis, Hothorn, Hornik (2005) and some illustrations are provided in `vignette("MOB")`.

For the fitted MOB tree, several standard methods are inherited if they are available for fitted `model`s, such as `print`, `predict`, `residuals`, `logLik`, `deviance`, `weights`, `coef` and `summary`. By default, the latter four return the result (deviance, weights, coefficients, summary) for all terminal nodes, but take a `node` argument that can be set to any node ID. The `sctest` method extracts the results of the parameter stability tests (aka structural change tests) for any given node, by default for all nodes. Some examples are given below.

### Value

An object of class `mob` inheriting from `BinaryTree-class`. Every node of the tree is additionally associated with a fitted model.

### References

Achim Zeileis, Torsten Hothorn and Kurt Hornik (2005). Model-based Recursive Partitioning. Report 19, Department of Statistics and Mathematics, Wirtschaftsuniversität Wien, Research Report Series. http://epub.wu-wien.ac.at/dyn/openURL?id=oai:epub-wu-01_86e

### See Also

`plot.mob`, `mob_control`

### Examples

    if(require("mlbench")) {

    ## recursive partitioning of a linear regression model
    ## load data
    data("BostonHousing", package = "mlbench")
    ## and transform variables appropriately (for a linear regression)
    BostonHousing$lstat <- log(BostonHousing$lstat)
    BostonHousing$rm <- BostonHousing$rm^2
    ## as well as partitioning variables (for fluctuation testing)
    BostonHousing$chas <- factor(BostonHousing$chas, levels = 0:1, labels = c("no", "yes"))
    BostonHousing$rad <- factor(BostonHousing$rad, ordered = TRUE)

    ## partition the linear regression model medv ~ lstat + rm
    ## with respect to all remaining variables:
    fmBH <- mob(medv ~ lstat + rm | zn + indus + chas + nox + age + dis + rad + tax + crim + b + ptratio,
      control = mob_control(minsplit = 40), data = BostonHousing, model = linearModel)

    ## print the resulting tree
    fmBH
    ## or better visualize it
    plot(fmBH)

    ## extract coefficients in all terminal nodes
    coef(fmBH)
    ## look at full summary, e.g., for node 7
    summary(fmBH, node = 7)
    ## results of parameter stability tests for that node
    sctest(fmBH, node = 7)
    ## -> no further significant instabilities (at 5% level)

    ## compute mean squared error (on training data)
    mean((BostonHousing$medv - fitted(fmBH))^2)
    mean(residuals(fmBH)^2)
    deviance(fmBH)/sum(weights(fmBH))

    ## evaluate logLik and AIC
    logLik(fmBH)
    AIC(fmBH)
    ## (Note that this penalizes estimation of error variances, which
    ## were treated as nuisance parameters in the fitting process.)

    ## recursive partitioning of a logistic regression model
    ## load data
    data("PimaIndiansDiabetes", package = "mlbench")
    ## partition logistic regression diabetes ~ glucose
    ## wth respect to all remaining variables
    fmPID <- mob(diabetes ~ glucose | pregnant + pressure + triceps + insulin + mass + pedigree + age,
      data = PimaIndiansDiabetes, model = glinearModel, family = binomial())

    ## fitted model
    coef(fmPID)
    plot(fmPID)
    plot(fmPID, tp_args = list(cdplot = TRUE))
    }

[Package party version 0.8-6 Index]