Introduction: We first take an intuitive look at how bagging works as a method of increasing accuracy. For ease of explanation, we will assume at first that our model is a classifier. Suppose that you are a patient and would like to have a diagnosis made based on your symptoms. Instead of asking one doctor, you may choose to ask several. If a certain diagnosis occurs more than any of the others, you may choose this as the final or best diagnosis. That is, the final diagnosis is made based on a majority vote, where each doctor gets an equal vote. Now replace each doctor by a classifier, and you have the basic idea behind bagging. Intuitively, a majority vote made by a large group of doctors may be more reliable than a majority vote made by a small group.
Given a set, D, of d tuples, bagging works as follows. For iteration i (i = 1, 2, : : : , k), a training set, Di, of d tuples is sampled with replacement from the original set of tuples, D. Note that the term bagging stands for bootstrap aggregation. Each training set is a bootstrap sample, as described in Section 6.13.3. Because sampling with replacement is used, some
Algorithm: Bagging. The bagging algorithm—creates an ensemble of models (classifiers or predictors) for a learning scheme where each model gives an equally-weighted prediction.
Input:
Output: A composite model, M*.
Method:
(1) for i = 1 to k do // create k models:
(2) create bootstrap sample, Di, by sampling D with replacement;
(3) use Di to derive a model, Mi;
(4) end for
To use the composite model on a tuple, X:
(1) if classification then
(2) let each of the k models classify X and return the majority vote;
(3) if prediction then
(4) let each of the k models predict a value for X and return the average predicted value;
of the original tuples of D may not be included in Di, whereas others may occur more than once. A classifier model, Mi, is learned for each training set, Di. To classify an unknown tuple, X, each classifier, Mi, returns its class prediction, which counts as one vote. The bagged classifier, M_, counts the votes and assigns the class with the most votes to X. Bagging can be applied to the prediction of continuous values by taking the average value of each prediction for a given test tuple. The algorithm is summarized in Figure 6.31.
The bagged classifier often has significantly greater accuracy than a single classifier derived from D, the original training data. It will not be considerably worse and is more robust to the effects of noisy data. The increased accuracy occurs because the composite model reduces the variance of the individual classifiers. For prediction, it was theoretically proven that a bagged predictor will always have improved accuracy over a single predictor derived from D.
Boosting: We now look at the ensemble method of boosting. As in the previous section, suppose that as a patient, you have certain symptoms. Instead of consulting one doctor, you choose to consult several. Suppose you assign weights to the value or worth of each doctor’s diagnosis, based on the accuracies of previous diagnoses they have made. The final diagnosis is then a combination of the weighted diagnoses. This is the essence behind boosting.
In boosting, weights are assigned to each training tuple. A series of k classifiers is iteratively learned. After a classifier Mi is learned, the weights are updated to allow the subsequent classifier, Mi 1, to “pay more attention” to the training tuples that were misclassified by Mi. The final boosted classifier, M*, combines the votes of each individual classifier, where the weight of each classifier’s vote is a function of its accuracy. The boosting algorithm can be extended for the prediction of continuous values.
