5.2.4 Decision Trees

Section Overview

A decision tree is a model made of questions. It is easy to read because each prediction follows a path of rules, but it can also overfit quickly when the rules become too detailed.

What You Will Build

In this lesson you will run one script that shows:

• how tree depth changes train/test accuracy;
• how to print readable tree rules;
• how feature importance is computed from splits;
• how post-pruning with ccp_alpha changes the number of leaves;
• how a regression tree makes step-like numeric predictions.

Read the maps first. A tree is not just "if-else"; it is "if-else plus a scoring rule plus complexity control."

Setup

python -m pip install -U scikit-learn

This lesson uses sklearn's CART-style DecisionTreeClassifier and DecisionTreeRegressor. CART means Classification and Regression Trees: the same tree idea can handle both labels and numbers.

Run the Complete Lab

Create decision_tree_lab.py:

from sklearn.datasets import load_diabetes, load_iris
from sklearn.metrics import accuracy_score, mean_absolute_error
from sklearn.model_selection import train_test_split
from sklearn.tree import DecisionTreeClassifier, DecisionTreeRegressor, export_text


iris = load_iris()
X = iris.data[:, 2:4]  # petal length and petal width, easier to read
y = iris.target
X_train, X_test, y_train, y_test = train_test_split(
    X, y, test_size=0.25, random_state=42, stratify=y
)

print("classification_depth_lab")
for depth in [1, 2, 3, None]:
    tree = DecisionTreeClassifier(max_depth=depth, min_samples_leaf=3, random_state=42)
    tree.fit(X_train, y_train)
    train_acc = accuracy_score(y_train, tree.predict(X_train))
    test_acc = accuracy_score(y_test, tree.predict(X_test))
    print(
        f"max_depth={str(depth):<4} "
        f"train={train_acc:.3f} test={test_acc:.3f} "
        f"leaves={tree.get_n_leaves()} depth={tree.get_depth()}"
    )

best_tree = DecisionTreeClassifier(max_depth=3, min_samples_leaf=3, random_state=42)
best_tree.fit(X_train, y_train)
print("tree_rules")
print(export_text(best_tree, feature_names=["petal length", "petal width"], decimals=2, max_depth=3))

print("feature_importance")
for name, value in zip(["petal length", "petal width"], best_tree.feature_importances_):
    print(f"- {name}: {value:.3f}")

print("pruning_lab")
path = DecisionTreeClassifier(random_state=42).cost_complexity_pruning_path(X_train, y_train)
for alpha in path.ccp_alphas[[0, 1, -2]]:
    pruned = DecisionTreeClassifier(random_state=42, ccp_alpha=float(alpha))
    pruned.fit(X_train, y_train)
    print(
        f"ccp_alpha={alpha:.4f} "
        f"test={accuracy_score(y_test, pruned.predict(X_test)):.3f} "
        f"leaves={pruned.get_n_leaves()}"
    )

print("regression_tree_lab")
diabetes = load_diabetes()
X_train, X_test, y_train, y_test = train_test_split(
    diabetes.data, diabetes.target, test_size=0.25, random_state=42
)
for depth in [2, 4, None]:
    reg = DecisionTreeRegressor(max_depth=depth, min_samples_leaf=10, random_state=42)
    reg.fit(X_train, y_train)
    pred = reg.predict(X_test)
    print(f"max_depth={str(depth):<4} mae={mean_absolute_error(y_test, pred):.1f} leaves={reg.get_n_leaves()}")

Run it:

python decision_tree_lab.py

Expected output:

classification_depth_lab
max_depth=1    train=0.670 test=0.658 leaves=2 depth=1
max_depth=2    train=0.964 test=0.947 leaves=3 depth=2
max_depth=3    train=0.982 test=0.974 leaves=5 depth=3
max_depth=None train=0.982 test=0.974 leaves=5 depth=3
tree_rules
|--- petal length <= 2.45
|   |--- class: 0
|--- petal length >  2.45
|   |--- petal width <= 1.70
|   |   |--- petal length <= 4.95
|   |   |   |--- class: 1
|   |   |--- petal length >  4.95
|   |   |   |--- class: 2
|   |--- petal width >  1.70
|   |   |--- petal length <= 4.95
|   |   |   |--- class: 2
|   |   |--- petal length >  4.95
|   |   |   |--- class: 2

feature_importance
- petal length: 0.588
- petal width: 0.412
pruning_lab
ccp_alpha=0.0000 test=0.921 leaves=7
ccp_alpha=0.0067 test=0.921 leaves=5
ccp_alpha=0.2636 test=0.658 leaves=2
regression_tree_lab
max_depth=2    mae=47.3 leaves=4
max_depth=4    mae=44.4 leaves=14
max_depth=None mae=48.7 leaves=25

Read the Output

The first block is the most important part:

max_depth=1    train=0.670 test=0.658 leaves=2 depth=1
max_depth=3    train=0.982 test=0.974 leaves=5 depth=3

max_depth=1 asks only one question, so the model is too simple. max_depth=3 asks a few follow-up questions and performs much better. On this tiny dataset, max_depth=None does not grow deeper because min_samples_leaf=3 and the data are already simple enough.

At each node, the tree searches for a question like:

petal length <= 2.45?

The chosen split should make the child nodes cleaner than the parent node. Cleaner means the labels inside a node are less mixed.

Gini, Entropy, and Information Gain

You do not need to memorize every formula on first pass. Remember the job:

Term	Practical meaning
Gini	How mixed the labels are in a node; sklearn's default for classification trees
entropy	Another mixedness score, connected to information theory
information gain	How much mixedness drops after a split
criterion	The setting that chooses the scoring rule, such as criterion="gini" or criterion="entropy"

Use gini first unless you have a reason to compare. In many tabular projects, tuning depth, leaf size, and pruning matters more than switching from Gini to entropy.

Complexity Control

The practical tuning order is:

1. Set max_depth to stop the tree from growing too deep.
2. Set min_samples_leaf so each leaf has enough examples.
3. Use ccp_alpha for post-pruning after a full tree is grown.

The pruning output shows the trade-off:

ccp_alpha=0.0000 test=0.921 leaves=7
ccp_alpha=0.0067 test=0.921 leaves=5
ccp_alpha=0.2636 test=0.658 leaves=2

A little pruning kept the same test score with fewer leaves. Too much pruning collapsed the tree into only two leaves and lost useful rules.

Interpretability

export_text() prints the path a sample will follow. This is useful when you need to explain a prediction to a teammate:

|--- petal length <= 2.45
|   |--- class: 0

Feature importance is also useful, but read it carefully:

• it tells you which features reduced impurity most in this fitted tree;
• it can favor features with many possible split points;
• correlated features can share or hide importance;
• it is not the same as causal importance.

For stronger interpretation, compare tree importance with permutation importance later.

Regression Trees

A regression tree predicts numbers, but the idea is the same: split the feature space into regions, then output the average target value in each leaf.

That is why regression tree predictions often look like steps rather than a smooth line. In the lab:

max_depth=4    mae=44.4 leaves=14
max_depth=None mae=48.7 leaves=25

The deeper tree has more leaves, but its test MAE is worse. More rules do not automatically mean better generalization.

When to Use a Single Decision Tree

Use a single tree when you need:

• a quick, explainable baseline;
• simple rule extraction for a business process;
• a visual way to explain non-linear splits;
• a stepping stone before Random Forest or boosting.

Avoid relying on a single tree when:

• small data changes create very different trees;
• test score drops far below train score;
• the problem needs the accuracy and stability of ensembles.

Practical Debugging Checklist

Symptom	Likely cause	Fix
Train score is high, test score is low	tree is too deep	lower max_depth, increase min_samples_leaf, try pruning
Tree has many tiny leaves	memorizing rare cases	increase min_samples_leaf
Feature importance feels misleading	correlated or high-cardinality features	verify with permutation importance
Rules are hard to read	tree is too large	train a smaller explanation tree, or summarize paths
Regression tree predictions look blocky	leaf averages create step outputs	compare with linear models, random forests, or gradient boosting

Practice

1. Change min_samples_leaf from 3 to 1, then to 10. What happens to leaves and test accuracy?
2. Change criterion to "entropy". Does the tree choose the same first split?
3. Print export_text() for a tree with max_depth=2. Is it easier to explain?
4. Replace the Iris features with all four features. Does feature importance change?
5. In the regression section, compare DecisionTreeRegressor with the linear regression lesson's baseline.

Pass Check

You are done when you can explain:

• a tree learns by choosing splits that make child nodes cleaner;
• deeper trees can memorize training data;
• max_depth, min_samples_leaf, and ccp_alpha control complexity;
• feature importance is useful but not automatically causal;
• regression trees output leaf averages, so predictions are step-like.