GBDT和XGBOOST算法原理
GBDT
以多分类问题为例介绍GBDT的算法，针对多分类问题，每次迭代都需要生成K个树（K为分类的个数），记为\(F_{mk}(x)\)，其中m为迭代次数，k为分类。
针对每个训练样本，使用的损失函数通常为$$L(y_i, F_{m1}(x_i), ..., F_{mK}(x_i))=-\sum_{k=1}{K}I({y_i}=k)ln[p_{mk}(x_i)]=-\sum_{k=1}I({y_i}=k)ln(\frac{e{F_{mk}(x_i)}}{\sum_{l=1}e^{F_{ml}(x_i)}})$$
此时损失函数的梯度可以表示为$$g_{mki}=-\frac{\partial{L(y_i, F_{m1}(x_i), ..., F_{mK}(x_i))}}{\partial{F_{mk}(x_i)}}=I({y_i}=k)-p_{mk}(x_i)=I({y_i}=k)-\frac{e{F_{mk}(x_i)}}{\sum_{l=1}e^{F_{ml}(x_i)}}$$
GBDT算法的流程如下所示：
- for k=1 to K: Initialize \(F_{0k}(x)=0\)
- for m=1 to M:
- for k=1 to K: compute \(g_{m-1,ki}\) for each sample \((x_i, y_i)\)
- for k=1 to K: build up regression tree \(R_{mkj}\)(j=1 to Jmk refer to the leaf nodes) from training samples \((x_i, g_{m-1,ki})_{i=1,...,N}\)
- for k=1 to K: compute leaf weights \(w_{mkj}\) for j=1 to Jmk
- for k=1 to K: \(F_{mk}(x)=F_{m-1,k}(x)+\eta*\sum_{j=1}^{J_{mk}}w_{mkj}I({x}\in{R_{mkj}})\), \(\eta\)为学习率
针对\(w_{mkj}\)的计算，有$$w_{mkj(j=1...J_{mk},k=1...K)}=argmin_{w_{kj(j=1...J_{mk},k=1...K)}}\sum_{i=1}^{N}L(y_i, ..., F_{m-1,k}(x_i)+\sum_{j=1}^{J_{mk}}w_{kj}I({x_i}\in{R_{mkj}}), ...)$$
为了求得w的值，使上述公式的一阶导数为0，利用Newton-Raphson公式（在这个问题中将初始值设为0，只进行一步迭代，并且Hessian矩阵只取对角线上的值），记\(L_i=L(y_i, ..., F_{m-1,k}(x_i)+\sum_{j=1}^{J_{mk}}w_{kj}I({x_i}\in{R_{mkj}}), ...)\)，有$$w_{mkj}=-\frac{\sum_{i=1}N\partial{L_i}/\partial{w_{kj}}}{\sum_{i=1}N\partial2{L_i}/\partial{w_{kj}2}}=\frac{\sum_{i=1}{N}I({x_i}\in{R_{mkj}})[I({y_i}=k)-p_{m-1,k}(x_i)]}{\sum_{i=1}I^2({x_i}\in{R_{mkj}})p_{m-1,k}(x_i)[1-p_{m-1,k}(x_i)]}=\frac{\sum_{x_i\in{R_{mkj}}}g_{m-1,ki}}{\sum_{x_i\in{R_{mkj}}}\lvert{g_{m-1,ki}}\rvert(1-\lvert{g_{m-1,ki}}\rvert)}$$
文献[1]在\(w_{mkj}\)的前面乘以了\(\frac{K-1}{K}\)的系数，可能原因是在分类器的建立过程中，可以把任意一个\(F_k(x)\)始终设为0，只计算剩余的\(F_k(x)\)，因此\(w_{mkj} \gets \frac{1}{K}*0+\frac{K-1}{K}w_{mkj}\)
参考文献
[1] Friedman, Jerome H. Greedy function approximation: A gradient boosting machine. Ann. Statist. 29 (2001), 1189--1232.
XGBOOST
仍以多分类问题介绍XGBOOST的算法，相对于GBDT的一阶导数，XGBOOST还引入了二阶导数，并且加入了正则项。
区别于上文的符号，记$$g_{mki}=\frac{\partial{L(y_i, F_{m1}(x_i), ..., F_{mK}(x_i))}}{\partial{F_{mk}(x_i)}}=p_{mk}(x_i)-I({y_i}=k)=\frac{e{F_{mk}(x_i)}}{\sum_{l=1}e{F_{ml}(x_i)}}-I({y_i}=k)$$以及$$h_{mki}=\frac{\partial2{L(y_i, F_{m1}(x_i), ..., F_{mK}(x_i))}}{\partial{F_{mk}(x_i)}^2}=p_{mk}(x_i)[1-p_{mk}(x_i)]$$
XGBOOST的计算流程如下：
- for k=1 to K: Initialize \(F_{0k}(x)=0\)
- for m=1 to M:
- for k=1 to K: compute \(g_{m-1,ki}\) and \(h_{m-1,ki}\) for each sample \((x_i, y_i)\)
- for k=1 to K: compute regression tree \(f_{mk}(x)=\sum_{j=1}^{J_{mk}}w_{mkj}I({x}\in{R_{mkj}})\), where \(R_{mkj}\)(j=1 to Jmk) refer to the leaf nodes, to minimize the function \(\sum_{i=1}^{N}[g_{m-1,ki}f_{mk}(x_i)+\frac{1}{2}h_{m-1,ki}f_{mk}(x_i)^2]+\Omega(f_{mk})\), where \(\Omega\) is the regularization function
- for k=1 to K: \(F_{mk}(x)=F_{m-1,k}(x)+\eta*\sum_{j=1}^{J_{mk}}w_{mkj}I({x}\in{R_{mkj}})\), \(\eta\)为学习率
针对回归树\(f_{mk}(x)=\sum_{j=1}^{J_{mk}}w_{mkj}I({x}\in{R_{mkj}})\)的求解，记\(\Omega(f_{mk})=\gamma J_{mk}+\frac \lambda {2}\sum_{j=1}^{J_{mk}}w_{mkj}^2\)，最小化问题变为Jmk个独立的二次函数之和：$$\sum_{j=1}^{J_{mk}}[(\sum_{x_i \in R_{mkj}}g_{m-1,ki})w_{mkj}+\frac{1}{2}(\lambda+\sum_{x_i \in R_{mkj}}h_{m-1,ki})w_{mkj}^2]+\gamma J_{mk}$$
假设回归树的结构已经固定，记\(T_k=J_{mk},G_{kj}=\sum_{x_i \in R_{mkj}}g_{m-1,ki}和H_{kj}=\sum_{x_i \in R_{mkj}}h_{m-1,ki}\)，此时在\(w_{mkj}=-\frac{G_{kj}}{H_{kj}+\lambda}\)时目标函数取得最小值，最小值为\(-\frac{1}{2}\sum_{j=1}^{T_k}\frac{G_{kj}^2}{H_{kj}+\lambda}+\gamma T_k\)
使用贪心算法确定回归树的结构，从一个节点开始不断进行分裂，分裂的规则是挑选使分裂增益最大的特征和分裂点进行分裂，直到达到某个停止条件为止。假设待分裂的节点为\(I\)，分裂后的两个节点分别为\(I_L\)和\(I_R\)，那么分裂增益可以表示为：$$-\frac{1}{2}\frac{(\sum_{x_i \in I}g_{m-1,ki})^2}{\lambda+\sum_{x_i \in I}h_{m-1,ki}}+\frac{1}{2}[\frac{(\sum_{x_i \in I_L}g_{m-1,ki})^2}{\lambda+\sum_{x_i \in I_L}h_{m-1,ki}}+\frac{(\sum_{x_i \in I_R}g_{m-1,ki})^2}{\lambda+\sum_{x_i \in I_R}h_{m-1,ki}}]-\gamma$$
XGBOOST的Python包中的重要参数
- eta(alias:learning_rate): learning rate or shrinkage，同上文的\(\eta\)，常用值0.01~0.2
- gamma(alias: min_split_loss): min loss reduction to create new tree split，同上文的\(\gamma\)
- lambda(alias: reg_lambda): L2 regularization on leaf weights，同上文的\(\lambda\)
- alpha(alias: reg_alpha): L1 regularization on leaf weights
- max_depth: max depth per tree，常用值3~10
- min_child_weight: minimum sum of instance weight(hessian) of all observations required in a child，即在一个节点中的样本的损失函数二阶导数之和，以多分类为例，\(\sum_{x_i \in I}h_{mki}=\sum_{x_i \in I}p_{mk}(x_i)[1-p_{mk}(x_i)]\)
- subsample: % samples used per tree，常用值0.5~1
- colsample_bytree: % features used per tree，常用值0.5~1
- 针对样本类别不均衡问题，若是二分类问题，可以设置参数scale_pos_weight，通常设为sum(negative instances)/sum(positive instances); 若是多分类问题，可以通过xgboost.DMatrix(...,weight,...)或者在Scikit-Learn API中调用fit(...,sample_weight,...)
- 特征重要性可以通过该特征被选中作为节点分裂特征的次数来度量
- xgboost应用示例可参见文章XGBOOST应用及调参示例
