Lesson 4

Hello and welcome to the **Decision Tree Models for Decision Making** lesson. We will be using the *Iris Dataset* and the *Sklearn* library in Python to understand the intricate universe of Decision Trees.

This lesson will enable you to grasp the basic concepts of training, implementing, and making predictions with decision tree models. In conclusion, you should have an in-depth understanding of how to implement decision tree models using the `Sklearn`

library in Python, how to train a decision tree model, and how to make predictions using the model.

We aim for a comprehensive grasp of decision tree models, from understanding the theory to implementing them practically on a real dataset. This enriched experience will undoubtedly boost your journey in the world of machine learning.

A **Decision Tree model** is a highly intuitive tool that uses a tree-like graph or model of decisions and their potential outcomes. It's essentially a structure similar to a flowchart, where each internal node denotes a test on an attribute, each branch represents the outcome of this test, and each leaf node (terminal node) holds a class label.

To help understand, think of decision tree models as tools for playing the game of "20 Questions". The game guesses what you're thinking by asking 20 'Yes' or 'No' questions. Each question progressively refines the possible answers, ultimately leading to the correct prediction.

In context, let's break down decision trees:

**Nodes**: These represent the attribute or feature that the model uses to make the decision.**Edges/Branches**: These signify the outcome of a decision, forming a link to the next decision.**Leaf Nodes**: These are the final outcomes or the predictions of the decision tree.

Now that we have the basics of decision tree models let's explore how to train them.

In machine learning, training a decision tree model involves furnishing it with a labeled dataset and allowing the model to learn the decisions based on these labels.

The method of training involves considering all attributes and their possible values to make the best split to divide the data. This `split`

is performed on the root node and all other internal nodes. It's based on metrics like entropy (impurity of labels), and information gain after splitting. Entropy controls how a Decision Tree decides where to split the data, making it one of the key factors in understanding how a Decision Tree works.

Let's imagine you want to predict whether it will rain based on the weather forecast. The model will split the data based on features like humidity, wind speed, and temperature, minimizing the entropy at each step.

Once trained, a decision tree model can be used to predict the outcome based on the given features. While the process of training involves node decisions on what feature to split, the prediction process will determine what outcome to take based on the feature value.

Consider this simple algorithm for prediction:

Python`1def predict(model, input): 2 node = model.root 3 while not node.is_leaf(): 4 if input[node.feature] <= node.threshold: 5 node = node.left_child 6 else: 7 node = node.right_child 8 return node.prediction`

This function takes an already trained `model`

and some input features. It starts at the `root`

of your decision tree, deciding whether to follow the left child node or the right child node based on the `threshold`

of the feature at that node. Then it repeats this process until it reaches a leaf node, which returns the final prediction.

Let's employ our understanding to create a decision tree model using Python and the `Sklearn`

library.

Begin by importing necessary libraries and preparing data:

Python`1from sklearn.datasets import load_iris 2from sklearn.model_selection import train_test_split 3from sklearn.tree import DecisionTreeClassifier 4 5# Import Iris dataset 6iris = load_iris() 7X = iris.data # features 8y = iris.target # Target variable`

We will further split our data for training and testing:

Python`1# Split the data into train and test sets 2X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=1)`

With the data prepared, let's train our decision tree model:

Python`1# Initialize the model 2clf = DecisionTreeClassifier() 3 4# Fit the model to the training data 5clf.fit(X_train, y_train)`

With the decision tree trained, we can now make predictions on our test set:

Python`1# Make predictions on the test set 2y_pred = clf.predict(X_test) 3 4# Print the predicted labels 5print("\nPredicted test labels:") 6print(y_pred)`

The output of this code will be:

`1Predicted test labels: 2[0 1 1 0 2 1 2 0 0 2 1 0 2 1 1 0 1 1 0 0 1 1 2 0 2 1 0 0 1 2]`

This output represents the predicted labels for our test set. We have successfully predicted the class of the iris flowers in our test set using a decision tree model.

In machine learning, it's not enough to just apply a model; we must also understand how to adjust it to perform better. Decision tree models have a number of key parameters that can be fine-tuned to improve their performance and make them more suited to specific datasets or scenarios.

One of the most important parameters is `max_depth`

, which controls the maximum depth of the tree. A deeper tree can model more complex relationships but may also lead to overfitting. Another parameter, `min_samples_split`

, determines the minimum number of samples required to split an internal node, and `min_samples_leaf`

specifies the minimum number of samples that must be left in a leaf node. Tweaking these parameters can help control the tree's growth and avoid overfitting.

Additionally, parameters like `max_features`

limit the number of features to consider when looking for the best split, and `criterion`

determines the function to measure the quality of a split (commonly `gini`

for Gini impurity or `entropy`

for information gain).

Here is a decision tree with some parameters set:

Python`1# Initialize the model with parameters 2clf_with_parameters = DecisionTreeClassifier(max_depth=3, min_samples_split=5, min_samples_leaf=4, max_features=2, criterion='entropy') 3 4# Fit the model to the training data 5clf_with_parameters.fit(X_train, y_train)`

After training your decision tree model with specific parameters, it's important to evaluate its performance. This brings us to the confusion matrix—a tool that gives you insights into how well your model is predicting.

A confusion matrix is a tabular representation of Actual vs Predicted values. Here's what it looks like:

Predicted No | Predicted Yes | |
---|---|---|

Actual No | TN | FP |

Actual Yes | FN | TP |

**True Positives (TP)**: The cases in which we predicted YES and the actual output was also YES.**True Negatives (TN)**: The cases in which we predicted NO and the actual output was NO.**False Positives (FP)**: The cases in which we predicted YES and the actual output was NO.**False Negatives (FN)**: The cases in which we predicted NO and the actual output was YES.

The confusion matrix is a powerful tool, as it allows you to calculate various performance metrics, like accuracy, precision, recall, and F1 score, which can tell you a lot about the strengths and weaknesses of your model.

To compute a confusion matrix in Python using Sklearn, you can use the following code:

Python`1from sklearn.metrics import confusion_matrix 2 3# Make predictions 4y_pred_parameters = clf_with_parameters.predict(X_test) 5 6# Compute confusion matrix 7cm = confusion_matrix(y_test, y_pred_parameters) 8 9# Display the confusion matrix 10print("Confusion Matrix:") 11print(cm)`

output

Python`1Confusion Matrix: 2[[11 0 0] 3 [ 0 12 1] 4 [ 0 0 6]]`

Understanding these parameters and metrics will greatly enhance your ability to create effective decision tree models and evaluate their performance. Keep experimenting with different settings in the practice section to find the best configuration for your specific problem—this hands-on experience is key to mastering machine learning algorithms!

Congratulations on completing this comprehensive course on Decision Tree Models. We have learned about decision tree models, trained one, used it to make predictions, and implemented it in Python using `Sklearn`

. You're now equipped with the skills to implement a fundamental machine learning model, which is a significant step towards advanced machine learning concepts.

Next, you have some exciting exercises where you'll get to incorporate what you've just learned. This hands-on practice will boost your understanding and confidence in decision tree models. Remember, skills improve with practice, so keep honing them. Let's continue our journey in machine learning!