adam2: Implementation of AdaBoost.M2

Description Usage Arguments Details Value References Examples

Description

The function implements AdaBoost.M2 for binary classification. It returns a list of weak learners that are built on random under-sampled training-sets, and a vector of error estimations of each weak learner. The weak learners altogether consist the ensemble model.

Usage

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adam2(formula, data, size, alg, rf.ntree = 50, svm.ker = "radial")

Arguments

formula

A formula specify predictors and target variable. Target variable should be a factor of 0 and 1. Predictors can be either numerical and categorical.

data

A data frame used for training the model, i.e. training set.

size

Ensemble size, i.e. number of weak learners in the ensemble model.

alg

The learning algorithm used to train weak learners in the ensemble model. cart, c50, rf, nb, and svm are available. Please see Details for more information.

rf.ntree

Number of decision trees in each forest of the ensemble model when using rf (Random Forest) as base learner. Integer is required.

svm.ker

Specifying kernel function when using svm as base algorithm. Four options are available: linear, polynomial, radial, and sigmoid. Default is radial. Equivalent to that in e1071::svm().

Details

AdaBoost.M2 is an extension of AdaBoost. AdaBoost.M2 introduces pseudo-loss, which is a more sophisticated method to estimate error and update instance weight in each iteration compared to AdaBoost and AdaBoost.M1. Although AdaBoost.M2 is originally implemented with decision tree, this function makes it possible to use other learning algorithms for building weak learners.

Argument alg specifies the learning algorithm used to train weak learners within the ensemble model. Totally five algorithms are implemented: cart (Classification and Regression Tree), c50 (C5.0 Decision Tree), rf (Random Forest), nb (Naive Bayes), and svm (Support Vector Machine). When using Random Forest as base learner, the ensemble model is consisted of forests and each forest contains a number of trees.

The function requires the target varible to be a factor of 0 and 1, where 1 indicates minority while 0 indicates majority instances. Only binary classification is implemented in this version.

The object class of returned list is defined as modelBst, which can be directly passed to predict() for predicting test instances.

Value

The function returns a list containing two elements:

weakLearners

A list of weak learners.

errorEstimation

Error estimation of each weak learner. Calculated by using (pseudo_loss + smooth) / (1 - pseudo_loss + smooth). smooth helps prevent error rate = 0 resulted from perfect classfication during trainging iterations. For more information, please see Schapire et al. (1999) Section 4.2.

References

Freund, Y. and Schapire, R. 1997. A Decision-Theoretic Generalization of On-Line Learning and an Application to Boosting. Journal of Computer and System Sciences. 55, pp. 119-139.

Freund, Y. and Schapire, R. 1996. Experiments with a new boosting algorithm. Machine Learning: In Proceedings of the 13th International Conference. pp. 148-156

Schapire, R. and Singer, Y. 1999. Improved Boosting Algorithms Using Confidence-rated Predictions. Machine Learning. 37(3). pp. 297-336.

Galar, M., Fernandez, A., Barrenechea, E., Bustince, H., and Herrera, F. 2012. A Review on Ensembles for the Class Imbalance Problem: Bagging-, Boosting-, and Hybrid-Based Approaches. IEEE Transactions on Systems, Man, and Cybernetics, Part C (Applications and Reviews). 42(4), pp. 463-484.

Examples

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data("iris")
iris <- iris[1:70, ]
iris$Species <- factor(iris$Species, levels = c("setosa", "versicolor"), labels = c("0", "1"))
model1 <- adam2(Species ~ ., data = iris, size = 10, alg = "c50")
model2 <- adam2(Species ~ ., data = iris, size = 20, alg = "rf", rf.ntree = 100)
model3 <- adam2(Species ~ ., data = iris, size = 40, alg = "svm", svm.ker = "sigmoid")

Example output



ebmc documentation built on May 1, 2019, 9:23 p.m.

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