Analytic Solver Data Mining offers two powerful ensemble methods for use with all classification methods: bagging (bootstrap aggregating) and boosting. A third method, random trees, may only be applied to classification trees. Each classification method on their own can be used to find one model that results in good classifications of the new data. We can view the statistics and confusion matrices of the current classifier to see if our model is a good fit to the data, but how would we know if there is a better classifier just waiting to be found? The answer is – we don't. However, ensemble methods allow us to combine multiple “weak” classification models which, when taken together form a new, more accurate “strong” classification model. These methods work by creating multiple diverse classification models, by taking different samples of the original dataset, and then combining their outputs. (Outputs may be combined by several techniques for example, majority vote for classification and averaging for regression. This combination of models effectively reduces the variance in the “strong” model. The three different types of ensemble methods offered in Analytic Solver Data Mining (bagging, boosting, and random trees) differ on three items: 1.The selection of training data for each classifier or “weak” model, 2.How the “weak” models are generated and 3. How the outputs are combined. In all three methods, each “weak” model is trained on the entire training dataset to become proficient in some portion of the dataset.
Bagging, or bootstrap aggregating, was one of the first ensemble algorithms ever to be written. It is a simple algorithm, yet very effective. Bagging generates several training data sets by using random sampling with replacement (bootstrap sampling), applies the classification algorithm to each dataset, then takes the majority vote amongst the models to determine the classification of the new data. The biggest advantage of bagging is the relative ease that the algorithm can be parallelized which makes it a better selection for very large datasets.
Boosting, in comparison, builds a “strong” model by successively training models to concentrate on the misclassified records in previous models. Once completed, all classifiers are combined by a weighted majority vote. Analytic Solver Data Mining offers three different variations of boosting as implemented by the AdaBoost algorithm (one of the most popular ensemble algorithms in use today): M1 (Freund), M1 (Breiman), and SAMME (Stagewise Additive Modeling using a Multi-class Exponential).
Adaboost.M1 first assigns a weight (wb(i)) to each record or observation. This weight is originally set to 1/n and will be updated on each iteration of the algorithm. An original classification model is created using this first training set (Tb) and an error is calculated as:
where the I() function returns 1 if true and 0 if not.
The error of the classification model in the bth iteration is used to calculate the constant αb. This constant is used to update the weight wb(i). In AdaBoost.M1 (Freund), the constant is calculated as:
In AdaBoost.M1 (Breiman), the constant is calculated as:
In SAMME, the constant is calculated as:
αb= 1/2ln((1-eb)/eb + ln(k-1) where k is the number of classes
(When the number of categories is equal to 2, SAMME behaves the same as AdaBoost Breiman.)
In any of the three implementations (Freund, Breiman, or SAMME), the new weight for the (b + 1)th iteration will be
Afterwards, the weights are all readjusted to sum to 1. As a result, the weights assigned to the observations that were classified incorrectly are increased and the weights assigned to the observations that were classified correctly are decreased. This adjustment forces the next classification model to put more emphasis on the records that were misclassified. (This α constant is also used in the final calculation which will give the classification model with the lowest error more influence.) This process repeats until b = Number of weak learners (controlled by the User). The algorithm then computes the weighted sum of votes for each class and assigns the “winning” classification to the record. Boosting generally yields better models than bagging, however, it does have a disadvantage as it is not parallelizable. As a result, if the number of weak learners is large, boosting would not be suitable.
Random trees, also known as random forests, is a variation of bagging. This method works by training multiple “weak” classification trees using a fixed number of randomly selected features (sqrt[number of features] for classification and number of features/3 for prediction) then takes the mode of each class to create a “strong” classifier. Typically, in this method the number of “weak” trees generated could range from several hundred to several thousand depending on the size and difficulty of the training set. Random Trees are parallelizable since they are a variant of bagging. However, since Random Trees selects a limited amount of features in each iteration, the performance of random trees is faster than bagging.
Classification Ensemble methods are very powerful methods and typically result in better performance than a single tree. This feature addition in Analytic Solver Data Mining (introduced in V2015) will provide users with more accurate classification models and should be considered.