# Decision Trees ALWAYS Overfit! Here's a Neat Technique to Prevent It

### Balancing cost and model size.

By default, a decision tree (in sklearn’s implementation, for instance), is allowed to grow until all leaves are pure.

This happens because a standard decision tree algorithm greedily selects the best split at each node.

This makes its nodes more and more pure as we traverse down the tree.

As the model correctly classifies ALL training instances, it leads to 100% overfitting, and poor generalization.

For instance, consider this dummy dataset:

Fitting a decision tree on this dataset gives us the following decision region plot:

It is pretty evident from the decision region plot, the training and test accuracy that the model has entirely overfitted our dataset.

**Cost-complexity-pruning (CCP)** is an effective technique to prevent this.

CCP considers a combination of two factors for pruning a decision tree:

Cost (C): Number of misclassifications

Complexity (C): Number of nodes

The core idea is to iteratively drop sub-trees, which, after removal, lead to:

a minimal increase in classification cost

a maximum reduction of complexity (or nodes)

In other words, if two sub-trees lead to a similar increase in classification cost, then it is wise to remove the sub-tree with more nodes.

In sklearn, you can control cost-complexity-pruning using the `ccp_alpha`

parameter:

large value of

`ccp_alpha`

→ results in underfittingsmall value of

`ccp_alpha`

→ results in overfitting

The objective is to determine the optimal value of ccp_alpha, which gives a better model.

The effectiveness of cost-complexity-pruning is evident from the image below:

As depicted above, CCP results in a much simpler and acceptable decision region plot.

That said, Bagging is another pretty effective way to avoid this overfitting problem.

The idea (as you may already know) is to:

create different subsets of data

**with replacement**(this is called bootstrapping)train one model per subset

aggregate all predictions to get the final prediction

As a result, it drastically reduces the variance of a single decision tree model, as shown below:

While we can indeed verify its effectiveness experimentally (as shown above), most folks struggle to intuitively understand:

**Why**Bagging is so effective.**Why**do we sample rows from the training dataset**with replacement**.How to mathematically formulate the idea of Bagging and prove variance reduction.

Can you answer these questions?

If not, we covered this in full detail here: **Why Bagging is So Ridiculously Effective At Variance Reduction?**

The article dives into the entire mathematical foundation of Bagging, which will help you:

Truly understand and appreciate the mathematical beauty of Bagging as an effective variance reduction technique

Why the random forest model is designed the way it is.

👉 Over to you: What are some other ways you use to prevent decision trees from overfitting?

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