1. What are Ensembles?
- Ensemble methods
combine multiple machine learning models to create more powerful and
robust models.
- By aggregating the predictions of many models, ensembles
typically achieve better generalization performance than any single model.
- In the context of decision trees, ensembles combine multiple trees
to overcome limitations of single trees such as overfitting and
instability.
2. Why Ensemble Decision Trees?
Single decision trees:
- Are easy to interpret but tend to overfit training data,
leading to poor generalization,.
- Can be unstable because small variations in data can
change the structure of the tree significantly.
Ensemble methods exploit the idea
that many weak learners (trees that individually overfit or only capture
partial patterns) can be combined to form a strong learner by reducing variance
and sometimes bias.
3. Two Main Types of Tree
Ensembles
(a) Random Forests
- Random forests are ensembles consisting of many decision
trees.
- Each tree is built on a bootstrap sample of the training data
(sampling with replacement).
- At each split in a tree, only a random subset of features
is considered for splitting.
- The aggregated prediction over all trees (majority vote
for classification, average for regression) reduces overfitting by
averaging diverse trees.
Key details:
- Randomness ensures the trees differ; otherwise,
correlated trees wouldn't reduce variance.
- Trees grown are typically deeper than single decision
trees because the random feature selection introduces diversity.
- Random forests are powerful out-of-the-box models
requiring minimal parameter tuning and usually do not require feature scaling.
(b) Gradient Boosted Decision
Trees
- Build trees sequentially,
where each new tree tries to correct errors of the combined ensemble built
so far.
- Unlike random forests which average predictions, gradient
boosting fits trees to the gradient
of a loss function to gradually improve predictiveness.
- This process often yields higher accuracy than random
forests but training is more computationally intensive and sensitive to
overfitting.
4. How Random Forests Inject
Randomness
- Data Sampling:
Bootstrap sampling ensures each tree is trained on a different subset of
data.
- Feature Sampling:
Each split considers only a subset of features randomly selected.
These two layers of randomness
ensure:
- Individual trees are less correlated.
- Averaging predictions reduces variance and prevents
overfitting seen in single deep trees.
5. Strengths of Ensembles of
Trees
- Robustness and accuracy:
Reduced overfitting due to averaging or boosting.
- Minimal assumptions:
Like single trees, ensembles typically do not require feature scaling or
extensive preprocessing.
- Handle large feature spaces and data:
Random forests can parallelize tree building and scale well.
- Feature importance:
Ensembles can provide measures of feature importance from aggregated
trees.
6. Weaknesses and Considerations
- Interpretability:
Ensembles lose the straightforward interpretability of single trees.
Hundreds of trees are hard to visualize and explain.
- Computational cost:
Training a large number of trees, especially with gradient boosting, can
be time-consuming.
- Parameter tuning:
Gradient boosting requires careful tuning (learning rate, tree depth,
number of trees) to avoid overfitting.
7. Summary Table for Random
Forests and Gradient Boosting
|
|
|
|
Tree
construction |
Parallel,
independent bootstrap samples |
Sequential,
residual fitting |
|
Randomness |
Data +
feature sampling |
Deterministic,
based on gradients |
|
Overfitting
control |
Averaging
many decorrelated trees |
Regularization,
early stopping, shrinkage |
|
Interpretability |
Lower
than single trees but feature importance available |
Lower;
complex, but feature importance measurable |
|
Computation |
Parallelizable;
faster |
Slower;
sequential |
|
Typical
use cases |
General-purpose,
robust models |
Performance-critical
tasks, often winning in competitions |
8. Additional Notes
- Both methods build on the decision tree structure explained in detail,.
- Random forests are often preferred as a baseline for
structured data due to simplicity and effectiveness.
- Gradient boosted trees can outperform random forests when
carefully tuned but are less forgiving.
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