Dietterich 2008 gradient tree boosting for training conditional random fields
Citation
Gradient Tree Boosting for Training Conditional Random Fields. By T. G Dietterich, G. Hao, A. Ashenfelter. In Journal of Machine Learning Research, vol. 9 ({{{issue}}}), 2008.
Online Version
This Paper is available online [1].
Summary
The paper addresses the problem of combinatorial explosion of parameters of CRFs when new features are introduced. It represents the potential functions as sums of regression trees. The authors claim that adding a regression tree is a big step in the feature space and hence it reduces the number of iterations. This leads to a significant performance improvement.
Method
In a traditional gradient descent method, a function is represented by linear function of parameters. Thus, the parameter values after steps would be . Instead of parameterizing, functional gradient ascent assumes that is a weighted sum of functions; , where
Since the joint distribution is not known, is empirically calculated from the training samples (which are assumed to be from the joint distribution. Now, a regression tree is trained to minimize the squared error loss. Overfitting is prevented by using a shrinkage parameter instead of bounding the number of leaf nodes of the tree.