bn.cv {bnlearn} | R Documentation

## Cross-validation for Bayesian networks

### Description

Perform a k-fold or hold-out cross-validation for a learning algorithm or a fixed network structure.

### Usage

    bn.cv(data, bn, loss = NULL, ..., algorithm.args = list(),
      loss.args = list(), fit = "mle", fit.args = list(), method = "k-fold",
      cluster = NULL, debug = FALSE)

    ## S3 method for class 'bn.kcv'
    plot(x, ..., main, xlab, ylab, connect = FALSE)
    ## S3 method for class 'bn.kcv.list'
    plot(x, ..., main, xlab, ylab, connect = FALSE)

    loss(x)

### Arguments

`data` | a data frame containing the variables in the model.
`bn` | either a character string (the label of the learning algorithm to be applied to the training data in each iteration) or an object of class `bn` (a fixed network structure).
`loss` | a character string, the label of a loss function. If none is specified, the default loss function is the Classification Error for Bayesian networks classifiers; otherwise, the Log-Likelihood Loss for both discrete and continuous data sets. See below for additional details.
`algorithm.args` | a list of extra arguments to be passed to the learning algorithm.
`loss.args` | a list of extra arguments to be passed to the loss function specified by `loss`.
`fit` | a character string, the label of the method used to fit the parameters of the newtork. See `bn.fit` for details.
`fit.args` | additional arguments for the parameter estimation procedure, see again `bn.fit` for details.
`method` | a character string, either `k-fold`, `custom-folds` or `hold-out`. See below for details.
`cluster` | an optional cluster object from package parallel.
`debug` | a boolean value. If `TRUE` a lot of debugging output is printed; otherwise the function is completely silent.
`x` | an object of class `bn.kcv` or `bn.kcv.list` returned by `bn.cv()`.
`...` | additional objects of class `bn.kcv` or `bn.kcv.list` to plot alongside the first.
`main, xlab, ylab` | the title of the plot, an array of labels for the boxplot, the label for the y axis.
`connect` | a logical value. If `TRUE`, the medians points in the boxplots will be connected by a segmented line.

### Value

`bn.cv()` returns an object of class `bn.kcv.list` if `runs` is at least 2, an object of class `bn.kcv` if `runs` is equal to 1.

`loss()` returns a numeric vector with a length equal to `runs`.

### Cross-Validation Strategies

The following cross-validation methods are implemented:

- k-fold: the `data` are split in `k` subsets of equal size. For each subset in turn, `bn` is fitted (and possibly learned as well) on the other `k - 1` subsets and the loss function is then computed using that subset. Loss estimates for each of the `k` subsets are then combined to give an overall loss for `data`.

- custom-folds: the data are manually partitioned by the user into subsets, which are then used as in k-fold cross-validation. Subsets are not constrained to have the same size, and every observation must be assigned to one subset.

- hold-out: `k` subsamples of size `m` are sampled independently without replacement from the `data`. For each subsample, `bn` is fitted (and possibly learned) on the remaining `m - nrow(data)` samples and the loss function is computed on the `m` observations in the subsample. The overall loss estimate is the average of the `k` loss estimates from the subsamples.

If either cross-validation is used with multiple `runs`, the overall loss is the averge of the loss estimates from the different runs.

To clarify, cross-validation methods accept the following optional arguments:

- `k`: a positive integer number, the number of groups into which the data will be split (in k-fold cross-validation) or the number of times the data will be split in training and test samples (in hold-out cross-validation).

- `m`: a positive integer number, the size of the test set in hold-out cross-validation.

- `runs`: a positive integer number, the number of times k-fold or hold-out cross-validation will be run.

- `folds`: a list in which element corresponds to one fold and contains the indices for the observations that are included to that fold; or a list with an element for each run, in which each element is itself a list of the folds to be used for that run.

### Loss Functions

The following loss functions are implemented:

- Log-Likelihood Loss (`logl`): also known as negative entropy or negentropy, it is the negated expected log-likelihood of the test set for the Bayesian network fitted from the training set.

- Gaussian Log-Likelihood Loss (`logl-g`): the negated expected log-likelihood for Gaussian Bayesian networks.

- Classification Error (`pred`): the prediction error for a single node in a discrete network. Frequentist predictions are used, so the values of the target node are predicted using only the information present in its local distribution (from its parents).

- Posterior Classification Error (`pred-lw` and `pred-lw-cg`): similar to the above, but predictions are computed from an arbitrary set of nodes using likelihood weighting to obtain Bayesian posterior estimates. `pred-lw` applies to discrete Bayesian networks, `pred-lw-cg` to (discrete nodes in) hybrid networks.

- Exact Classification Error (`pred-exact`): closed-form exact posterior predictions are available for Bayesian network classifiers.

- Predictive Correlation (`cor`): the correlation between the observed and the predicted values for a single node in a Gaussian Bayesian network.

- Posterior Predictive Correlation (`cor-lw` and `cor-lw-cg`): similar to the above, but predictions are computed from an arbitrary set of nodes using likelihood weighting to obtain Bayesian posterior estimates. `cor-lw` applies to Gaussian networks and `cor-lw-cg` to (continuous nodes in) hybrid networks.

- Mean Squared Error (`mse`): the mean squared error between the observed and the predicted values for a single node in a Gaussian Bayesian network.

- Posterior Mean Squared Error (`mse-lw` and `mse-lw-cg`): similar to the above, but predictions are computed from an arbitrary set of nodes using likelihood weighting to obtain Bayesian posterior estimates. `mse-lw` applies to Gaussian networks and `mse-lw-cg` to (continuous nodes in) hybrid networks.

Optional arguments that can be specified in `loss.args` are:

- `target`: a character string, the label of target node for prediction in all loss functions but `logl`, `logl-g` and `logl-cg`.

- `from`: a vector of character strings, the labels of the nodes used to predict the `target` node in `pred-lw`, `pred-lw-cg`, `cor-lw`, `cor-lw-cg`, `mse-lw` and `mse-lw-cg`. The default is to use all the other nodes in the network. Loss functions `pred`, `cor` and `mse` implicitly predict only from the parents of the `target` node.

- `n`: a positive integer, the number of particles used by likelihood weighting for `pred-lw`, `pred-lw-cg`, `cor-lw`, `cor-lw-cg`, `mse-lw` and `mse-lw-cg`. The default value is `500`.

Note that if `bn` is a Bayesian network classifier, `pred` and `pred-lw` both give exact posterior predictions computed using the closed-form formulas for naive Bayes and TAN.

### Plotting Results from Cross-Validation

Both plot methods accept any combination of objects of class `bn.kcv` or `bn.kcv.list` (the first as the `x` argument, the remaining as the `...` argument) and plot the respected expected loss values side by side. For a `bn.kcv` object, this mean a single point; for a `bn.kcv.list` object this means a boxplot.

### Author(s)

Marco Scutari

### References

Koller D, Friedman N (2009). Probabilistic Graphical Models: Principles and Techniques. MIT Press.

### See Also

`bn.boot`, `rbn`, `bn.kcv-class`.

### Examples

    bn.cv(learning.test, 'hc', loss = "pred", loss.args = list(target = "F"))

    folds = list(1:2000, 2001:3000, 3001:5000)
    bn.cv(learning.test, 'hc', loss = "logl", method = "custom-folds",
      folds = folds)

    xval = bn.cv(gaussian.test, 'mmhc', method = "hold-out",
             k = 5, m = 50, runs = 2)
    xval
    loss(xval)

    ## Not run:
    # comparing algorithms with multiple runs of cross-validation.
    gaussian.subset = gaussian.test[1:50, ]
    cv.gs = bn.cv(gaussian.subset, 'gs', runs = 10)
    cv.iamb = bn.cv(gaussian.subset, 'iamb', runs = 10)
    cv.inter = bn.cv(gaussian.subset, 'inter.iamb', runs = 10)
    plot(cv.gs, cv.iamb, cv.inter,
      xlab = c("Grow-Shrink", "IAMB", "Inter-IAMB"), connect = TRUE)

    # use custom folds.
    folds = split(sample(nrow(gaussian.subset)), seq(5))
    bn.cv(gaussian.subset, "hc", method = "custom-folds", folds = folds)

    # multiple runs, with custom folds.
    folds = replicate(5, split(sample(nrow(gaussian.subset)), seq(5)),
              simplify = FALSE)
    bn.cv(gaussian.subset, "hc", method = "custom-folds", folds = folds)

    ## End(Not run)

[Package bnlearn version 4.5 Index]