---
id: wiki-2026-0508-ridge-regression
title: Ridge Regression
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [L2 Regularization, Tikhonov Regularization]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [machine-learning, regression, regularization, statistics]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: Python
  framework: scikit-learn
---

# Ridge Regression

## 매 한 줄
> **"매 L2 penalty 의 OLS 의 stabilize"**. Hoerl & Kennard (1970) 의 multicollinearity 의 fix 의 introduce — 매 modern ML 의 baseline regularizer 의 사용 (sklearn `Ridge`, `RidgeCV`).

## 매 핵심

### 매 Loss function
- OLS: `min ||y - Xβ||²`
- Ridge: `min ||y - Xβ||² + α||β||²`
- α (alpha) → regularization strength. α=0 → OLS. α→∞ → β→0.

### 매 Closed form
- `β̂ = (XᵀX + αI)⁻¹ Xᵀy`
- `XᵀX + αI` 는 invertible — 매 multicollinear 한 X 도 OK.
- OLS 의 `(XᵀX)⁻¹` 는 singular 가능 → ridge 가 fix.

### 매 응용
1. Multicollinear features (correlated predictors).
2. p > n (features more than samples) — gene expression, fMRI.
3. Baseline 의 sklearn pipelines.

## 💻 패턴

### Sklearn Ridge
```python
from sklearn.linear_model import Ridge, RidgeCV
from sklearn.preprocessing import StandardScaler
from sklearn.pipeline import make_pipeline

# Always scale before ridge — penalty is scale-dependent
model = make_pipeline(StandardScaler(), Ridge(alpha=1.0))
model.fit(X_train, y_train)
print(model.score(X_test, y_test))
```

### Cross-validated alpha
```python
# RidgeCV picks best alpha from grid
ridge = RidgeCV(alphas=[0.01, 0.1, 1.0, 10.0, 100.0], cv=5)
ridge.fit(X_train, y_train)
print(f"Best alpha: {ridge.alpha_}")
```

### Closed-form by hand
```python
import numpy as np

def ridge_fit(X, y, alpha):
    n, p = X.shape
    I = np.eye(p)
    return np.linalg.solve(X.T @ X + alpha * I, X.T @ y)

beta = ridge_fit(X_train, y_train, alpha=1.0)
```

### SVD-based ridge (numerically stable)
```python
def ridge_svd(X, y, alpha):
    U, s, Vt = np.linalg.svd(X, full_matrices=False)
    d = s / (s**2 + alpha)
    return Vt.T @ (d * (U.T @ y))
```

### Kernel Ridge
```python
from sklearn.kernel_ridge import KernelRidge

# Non-linear ridge via kernel trick
krr = KernelRidge(alpha=1.0, kernel='rbf', gamma=0.1)
krr.fit(X_train, y_train)
```

### Bayesian view
```python
from sklearn.linear_model import BayesianRidge

# Ridge as Gaussian prior on β with variance 1/α
br = BayesianRidge()
br.fit(X_train, y_train)
print(br.coef_, br.alpha_)  # learned alpha
```

### Regularization path
```python
import numpy as np
from sklearn.linear_model import Ridge

alphas = np.logspace(-3, 3, 50)
coefs = []
for a in alphas:
    r = Ridge(alpha=a).fit(X, y)
    coefs.append(r.coef_)
# Plot coefs vs log(alpha) — see shrinkage
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Multicollinear features | Ridge |
| Need feature selection | Lasso (L1) |
| Mix of sparsity + grouping | Elastic Net |
| Non-linear pattern | Kernel Ridge |
| Bayesian uncertainty | BayesianRidge |

**기본값**: `RidgeCV` with log-spaced alpha grid + StandardScaler.

## 🔗 Graph
- 부모: [[Linear-Regression]] · [[Regularization]]
- 변형: [[Lasso-Regression]] · [[Elastic-Net]] · [[Kernel-Ridge]]
- 응용: [[Feature-Engineering]] · [[Bias-Variance-Tradeoff]]
- Adjacent: [[Singular-Value-Decomposition]] · [[Bayesian-Linear-Regression]]

## 🤖 LLM 활용
**언제**: Tabular regression 의 strong baseline. Multicollinear features (correlated predictors) 의 시. p > n 의 high-dim setting. Linear model interpretability 의 keep.
**언제 X**: Sparse feature selection 의 필요 (use Lasso). Strong non-linearity (use trees/NN). N >> p 의 와 features uncorrelated → OLS 도 충분.

## ❌ 안티패턴
- **No scaling**: Ridge penalty 의 scale-sensitive — features 의 다른 scale 의 → 매 unfair shrinkage.
- **Manual alpha pick**: 매 RidgeCV 의 use, magic number alpha=1.0 의 X.
- **Ridge for sparsity**: L2 의 X coefficient 의 zero 의 안 만든다 — Lasso 의 use.
- **Ignoring intercept**: sklearn 의 default 는 intercept 의 X regularize — but custom impl 의 watch.

## 🧪 검증 / 중복
- Verified (Hoerl & Kennard 1970, ESL Ch.3, sklearn docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Ridge regression with closed-form, SVD, kernel, Bayesian variants |
