Gradient Boosting Classification in Python

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Gradient Boosting is an alternative form of boosting to AdaBoost. Many consider gradient boosting to be a better performer than adaboost. Some differences between the two algorithms is that gradient boosting uses optimization for weight the estimators. Like adaboost, gradient boosting can be used for most algorithms but is commonly associated with decision trees.

In addition, gradient boosting requires several additional hyperparameters such as max depth and subsample. Max depth has to do with the number of nodes in a tree. The higher the number the purer the classification become. The downside to this is the risk of overfitting.

Subsampling has to do with the proportion of the sample that is used for each estimator. This can range from a decimal value up until the whole number 1. If the value is set to 1 it becomes stochastic gradient boosting.

This post is focused on classification. To do this, we will use the cancer dataset from the pydataset library. Our goal will be to predict the status of patients (alive or dead) using the available independent variables. The steps we will use are as follows.

  1. Data preparation
  2. Baseline decision tree model
  3. Hyperparameter tuning
  4. Gradient boosting model development

Below is some initial code.

from sklearn.ensemble import GradientBoostingClassifier
 from sklearn import tree
 from sklearn.model_selection import GridSearchCV
 import numpy as np
 from pydataset import data
 import pandas as pd
 from sklearn.model_selection import cross_val_score
 from sklearn.model_selection import KFold

Data Preparation

The data preparation is simple in this situtation. All we need to do is load are dataset, dropping missing values, and create our X dataset and y dataset. All this happens in the code below.

df=data('cancer').dropna()
 X=df[['time','sex','ph.karno','pat.karno','meal.cal','wt.loss']]
 y=df['status']

We will now develop our baseline decision tree model.

Baseline Model

The purpose of the baseline model is to have something to compare our gradient boosting model to. The strength of a model is always relative to some other model, so we need to make at least two, so we can say one is better than the other.

The criteria for better in this situation is accuracy. Therefore, we will make a decision tree model, but we will manipulate the max depth of the tree to create 9 different baseline models. The best accuracy model will be the baseline model.

To achieve this, we need to use a for loop to make python make several decision trees. We also need to set the parameters for the cross validation by calling KFold(). Once this is done, we print the results for the 9 trees. Below is the code and results.

crossvalidation=KFold(n_splits=10,shuffle=True,random_state=1)
for depth in range (1,10):
     tree_classifier=tree.DecisionTreeClassifier(max_depth=depth,random_state=1)
     if tree_classifier.fit(X,y).tree_.max_depth<depth:
         break
     score=np.mean(cross_val_score(tree_classifier,X,y,scoring='accuracy', cv=crossvalidation,n_jobs=1))
     print(depth, score)
 1 0.71875
 2 0.6477941176470589
 3 0.6768382352941177
 4 0.6698529411764707
 5 0.6584558823529412
 6 0.6525735294117647
 7 0.6283088235294118
 8 0.6573529411764706
 9 0.6577205882352941

It appears that when the max depth is limited to 1 that we get the best accuracy at almost 72%. This will be our baseline for comparison. We will now tune the parameters for the gradient boosting algorithm

Hyperparameter Tuning

There are several hyperparameters we need to tune. The ones we will tune are as follows

  • number of estimators
  • learning rate
  • subsample
  • max depth

First, we will create an instance of the gradient boosting classifier. Second, we will create our grid for the search. It is inside this grid that we set several values for each hyperparameter. Then we call GridSearchCV and place the instance of the gradient boosting classifier, the grid, the cross validation values from mad earlier, and n_jobs all together in one place. Below is the code for this.

GBC=GradientBoostingClassifier()
 search_grid={'n_estimators':[500,1000,2000],'learning_rate':[.001,0.01,.1],'max_depth':[1,3,5],'subsample':[.5,.75,1],'random_state':[1]}
search=GridSearchCV(estimator=GBC,param_grid=search_grid,scoring='accuracy',n_jobs=1,cv=crossvalidation)

You can now run your model by calling .fit(). Keep in mind that there are several hyperparameters. This means that it might take some time to run the calculations. It is common to find values for max depth, subsample, and number of estimators first. Then as second run through is done to find the learning rate. In our example, we are doing everything at once which is why it takes longer. Below is the code with the out for best parameters and best score.

search.fit(X,y)
search.best_params_
 Out[11]: 
 {'learning_rate': 0.01,
  'max_depth': 5,
  'n_estimators': 2000,
  'random_state': 1,
  'subsample': 0.75}
 search.best_score_
 Out[12]: 0.7425149700598802

You can see what the best hyperparameters are for yourself. In addition, we see that when these parameters were set we got an accuracy of 74%. This is superior to our baseline model. We will now see if we can replicate these numbers when we use them for our Gradient Boosting model.

Gradient Boosting Model

Below is the code and results for the model with the predetermined hyperparameter values.

ada2=GradientBoostingClassifier(n_estimators=2000,learning_rate=0.01,subsample=.75,max_depth=5,random_state=1)
 score=np.mean(cross_val_score(ada2,X,y,scoring='accuracy',cv=crossvalidation,n_jobs=1))
 score
 Out[17]: 0.742279411764706

You can see that the results are similar. This is just additional information that the gradient boosting model does outperform the baseline decision tree model.

Conclusion

This post provided an example of what gradient boosting classification can do for a model. With its distinct characteristics gradient boosting is generally a better performing boosting algorithm in comparison to AdaBoost.

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