Decision tree is a supervised machine learning algorithm that is used for both classification and regression tasks. It works by recursively partitioning the feature space into smaller and smaller sub-spaces based on the values of the input features until it reaches a stopping criterion.
Here is a step-by-step explanation of how the decision tree algorithm works:
- Data Preparation: Decision tree algorithms work with tabular data, where each row represents an observation or example and each column represents a feature or attribute of that observation. The data is split into a training set and a testing set.
- Feature Selection: The algorithm selects the most important feature to split the dataset into two subsets. This is done by calculating the information gain or gain ratio for each feature, which measures the reduction in uncertainty achieved by splitting on that feature.
- Splitting: The data is split based on the selected feature into two or more subsets. This process is repeated recursively for each subset until a stopping criterion is reached. The stopping criterion can be based on the maximum depth of the tree, the minimum number of samples required to split a node, or the minimum improvement in impurity achieved by the split.
- Prediction: Once the decision tree is constructed, it can be used to predict the class or value of new examples. This is done by traversing the tree from the root node to a leaf node that corresponds to the predicted class or value.
Here is an example of a decision tree for a binary classification problem:
In this example, we have a dataset of patients who have undergone a medical test for a particular disease. The dataset has three features: age, gender, and test result, and the target variable is whether the patient has the disease or not.
The decision tree algorithm selects the feature with the highest information gain, which in this case is the test result. The dataset is split into two subsets based on the test result: one subset for patients who tested positive and one subset for patients who tested negative.
For the subset of patients who tested positive, the algorithm selects the feature with the highest information gain, which is age. The subset is split into two subsets based on age: patients who are older than 50 and patients who are younger than 50.
For the subset of patients who tested negative, the algorithm selects the feature with the highest information gain, which is gender. The subset is split into two subsets based on gender: male patients and female patients.
The decision tree can be used to predict the target variable for new patients based on their age, gender, and test result. For example, if a patient is a female who tested negative and is younger than 50, the decision tree predicts that she does not have the disease.