When working with regression models in machine learning, two popular regularization techniques often come into play: Lasso Regression and Ridge Regression. While both techniques help mitigate overfitting by penalizing large coefficients, they behave very differently when it comes to creating sparsity in the model. In this article, we'll explore why Lasso regression can shrink some coefficients to zero, effectively performing feature selection, while Ridge regression does not exhibit this property.
Understanding Regularization
Regularization involves adding a penalty term to the cost function to discourage overly complex models. By penalizing large coefficients, regularization prevents the model from overfitting the training data, improving its ability to generalize to unseen data.
Lasso and Ridge regression differ in the type of penalty they apply:
- Lasso Regression uses the L1 norm as a penalty.
- Ridge Regression uses the L2 norm as a penalty
Why Ridge Does Not Create Sparsity
The L2 norm used in Ridge regression does not drive coefficients to exactly zero. Instead, it shrinks all coefficients proportionally, reducing their magnitude without eliminating any completely. Here's why:
- Circular Constraint Region: In high-dimensional space, the L2 penalty forms a circular constraint region. The optimization process intersects this region at points where all coefficients are small but typically nonzero.
- Smooth Penalty: The L2 penalty is smooth and differentiable everywhere, including at zero. Unlike the L1 penalty, it does not have the sharp corner that encourages exact zeros.
As a result, Ridge regression retains all features, even if their contributions are minimal, making it less suitable for feature selection.
Key Differences Between Lasso and Ridge Regression
| Feature | Lasso Regression | Ridge Regression |
|---|---|---|
| Penalty Type | L1 norm (( | \beta_j |
| Shrinking Coefficients | Can shrink to exactly 0 | Shrinks toward 0 but not exactly 0 |
| Sparsity | Yes (performs feature selection) | No (all features retained) |
| Constraint Region | Diamond-shaped | Circular |
When to Use Lasso vs. Ridge
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Use Lasso when:
- You expect many features to be irrelevant or redundant.
- Sparsity or feature selection is desired.
-
Use Ridge when:
- All features are likely to contribute to the target variable.
- You want to prevent overfitting without losing any features.
Conclusion
Lasso and Ridge regression are powerful tools for regularization, but they cater to different needs. Lasso's ability to create sparsity by setting coefficients to exactly zero makes it ideal for feature selection. Ridge, on the other hand, excels at shrinking coefficients uniformly, preserving all features. Understanding these differences enables you to choose the right technique for your machine learning model.
By leveraging these methods effectively, you can build more robust, interpretable, and generalizable models that suit your specific problem.
Have any questions or thoughts? Let’s discuss in the comments below!
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