随机过采样与 SMOTE
您将使用 imbalanced-learn 中的 RandomOverSampler 和 SMOTE 对少数类进行过采样，然后评估精确率和召回率是否得到改善。
随机过采样与 SMOTE 是 CoddyKit 上的免费 Machine Learning Academy 课时。 这是第 2 节课，共 4 节。 你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。 这是 Machine Learning Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Machine Learning Academy 课程共包含 4 节课。
本课时的部分内容尚未翻译，以英文显示。
Why Oversampling Helps Imbalanced Models
Many classifiers learn a biased decision boundary when training data is dominated by one class — they effectively ignore the minority class. Oversampling increases the representation of the minority class in the training data, forcing the model to pay more attention to it. Two popular approaches are random oversampling (duplicating existing minority samples) and SMOTE (synthesising new ones).
Random Oversampling: Duplicate Minority Samples
RandomOverSampler from the imbalanced-learn library randomly duplicates examples from the minority class until the desired class ratio is achieved. It is simple and effective, but the duplicate samples add no new information — the model may overfit to the repeated minority examples if not regularised carefully.
from imblearn.over_sampling import RandomOverSampler
from sklearn.datasets import make_classification
import numpy as np
X, y = make_classification(n_samples=1000, weights=[0.95, 0.05],
n_features=10, random_state=42)
print('Before resampling:', np.bincount(y))
ros = RandomOverSampler(random_state=42)
X_res, y_res = ros.fit_resample(X, y)
print('After resampling: ', np.bincount(y_res))Installing imbalanced-learn
imbalanced-learn is a scikit-learn compatible library for handling class imbalance. Install it with pip install imbalanced-learn. It follows the sklearn API — samplers have fit_resample instead of fit_transform. It integrates with sklearn Pipelines via imblearn.pipeline.Pipeline, which is a drop-in replacement that supports samplers as pipeline steps.
# pip install imbalanced-learn
from imblearn import __version__ as imb_version
from imblearn.over_sampling import RandomOverSampler, SMOTE
from imblearn.under_sampling import RandomUnderSampler
from imblearn.pipeline import Pipeline as ImbPipeline
print('imbalanced-learn version:', imb_version)SMOTE: Synthetic Minority Oversampling
SMOTE (Synthetic Minority Oversampling TEchnique) creates new synthetic minority examples rather than duplicating existing ones. For each minority sample, SMOTE finds its k nearest minority neighbours and creates new points along the line segments connecting them. These synthetic samples occupy the interior of the minority feature space, giving the classifier richer information about the minority class boundary.
SMOTE in Practice
Use imblearn.over_sampling.SMOTE. Key parameters: k_neighbors (default 5) — how many minority neighbours to consider. The sampling_strategy parameter controls the target ratio: 'auto' oversamples all minority classes to match the majority class count.
from imblearn.over_sampling import SMOTE
from sklearn.datasets import make_classification
import numpy as np
X, y = make_classification(n_samples=1000, weights=[0.95, 0.05],
n_features=10, random_state=42)
print('Before SMOTE:', np.bincount(y))
smote = SMOTE(k_neighbors=5, random_state=42)
X_res, y_res = smote.fit_resample(X, y)
print('After SMOTE:', np.bincount(y_res))
print('New shape:', X_res.shape)Comparing Random Oversampling vs SMOTE
Random oversampling simply copies existing data and is faster; SMOTE synthesises new data and tends to generalise better. However, SMOTE can create unrealistic samples if the minority class is very sparse or if features have complex interactions. On highly non-linear problems, borderline-SMOTE or ADASYN (Adaptive Synthetic Sampling) may work better than standard SMOTE.
from imblearn.over_sampling import RandomOverSampler, SMOTE
from sklearn.linear_model import LogisticRegression
from sklearn.metrics import f1_score
from sklearn.datasets import make_classification
from sklearn.model_selection import train_test_split
from sklearn.preprocessing import StandardScaler
import numpy as np
X, y = make_classification(n_samples=1000, weights=[0.95, 0.05], random_state=42)
X_train, X_test, y_train, y_test = train_test_split(X, y, stratify=y, random_state=42)
sc = StandardScaler()
X_train_s = sc.fit_transform(X_train)
X_test_s = sc.transform(X_test)
for name, sampler in [('None', None), ('ROS', RandomOverSampler(random_state=0)),
('SMOTE', SMOTE(random_state=0))]:
if sampler:
X_r, y_r = sampler.fit_resample(X_train_s, y_train)
else:
X_r, y_r = X_train_s, y_train
lr = LogisticRegression().fit(X_r, y_r)
f1 = f1_score(y_test, lr.predict(X_test_s))
print(f'{name:6s}: F1={f1:.4f}')Using imbalanced-learn Pipeline
The critical rule: apply SMOTE only to the training fold, never to the test fold. The imblearn.pipeline.Pipeline enforces this automatically — SMOTE's fit_resample is called during fit but not during predict or transform. This is why you must use imblearn.pipeline.Pipeline, not sklearn's, when including a sampler.
from imblearn.pipeline import Pipeline as ImbPipeline
from imblearn.over_sampling import SMOTE
from sklearn.preprocessing import StandardScaler
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import cross_val_score
from sklearn.datasets import make_classification
import numpy as np
X, y = make_classification(n_samples=1000, weights=[0.95, 0.05], random_state=42)
pipe = ImbPipeline([
('sc', StandardScaler()),
('smote', SMOTE(random_state=42)), # applied only during fit
('lr', LogisticRegression())
])
scores = cross_val_score(pipe, X, y, cv=5, scoring='f1')
print(f'CV F1: {np.mean(scores):.4f} +/- {np.std(scores):.4f}')Visualising SMOTE Samples
On a 2D dataset you can visualise where SMOTE places synthetic samples. The new points lie along line segments between existing minority samples in feature space. This confirms that SMOTE is interpolating within the minority class region — not extrapolating outside it.
import matplotlib.pyplot as plt
from imblearn.over_sampling import SMOTE
from sklearn.datasets import make_classification
X, y = make_classification(n_samples=200, weights=[0.9, 0.1],
n_features=2, n_redundant=0,
n_informative=2, random_state=42)
smote = SMOTE(random_state=0)
X_res, y_res = smote.fit_resample(X, y)
fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(10, 4))
for ax, data, labels, title in [(ax1, X, y, 'Before SMOTE'),
(ax2, X_res, y_res, 'After SMOTE')]:
ax.scatter(data[labels==0, 0], data[labels==0, 1], label='Class 0', alpha=0.5)
ax.scatter(data[labels==1, 0], data[labels==1, 1], label='Class 1', alpha=0.5)
ax.set_title(title)
ax.legend()
plt.tight_layout()
plt.show()SMOTE Variants: Borderline and ADASYN
Borderline-SMOTE generates synthetic samples only near the decision boundary (borderline minority samples closest to the majority class), focusing the model's attention where classification is hardest. ADASYN adaptively generates more samples in regions where the class is densest, adding proportionally more synthetic examples where the classifier struggles most.
from imblearn.over_sampling import BorderlineSMOTE, ADASYN
from sklearn.datasets import make_classification
import numpy as np
X, y = make_classification(n_samples=1000, weights=[0.95, 0.05], random_state=42)
print('Before:', np.bincount(y))
bs = BorderlineSMOTE(random_state=0)
X_bs, y_bs = bs.fit_resample(X, y)
print('After BorderlineSMOTE:', np.bincount(y_bs))
adas = ADASYN(random_state=0)
X_ad, y_ad = adas.fit_resample(X, y)
print('After ADASYN:', np.bincount(y_ad))Combining SMOTE with Undersampling (SMOTEENN)
SMOTE alone can introduce noisy samples near the majority-class boundary. SMOTEENN combines SMOTE with Edited Nearest Neighbours (ENN) cleaning: after SMOTE upsamples, ENN removes misclassified samples from both classes near the boundary. The result is a cleaner dataset with both oversampled minority and cleaned majority regions.
from imblearn.combine import SMOTEENN
from sklearn.datasets import make_classification
import numpy as np
X, y = make_classification(n_samples=1000, weights=[0.95, 0.05], random_state=42)
print('Before:', np.bincount(y))
smoteenn = SMOTEENN(random_state=42)
X_res, y_res = smoteenn.fit_resample(X, y)
print('After SMOTEENN:', np.bincount(y_res))Evaluating SMOTE: Precision-Recall Trade-Off
After applying oversampling, always evaluate with metrics sensitive to the minority class — F1-score, ROC-AUC, or PR-AUC. SMOTE typically improves recall (fewer missed positives) but may reduce precision (more false alarms). Use the precision-recall curve to find the threshold that gives your desired operating point for the specific business cost of false positives vs false negatives.
Quick Check
Test your understanding of oversampling and SMOTE from this lesson.
Lesson Recap
In this lesson you learned: RandomOverSampler duplicates minority examples — simple but risks overfitting to repeated data, SMOTE synthesises new minority samples by interpolating between existing ones, giving the classifier richer information, and imblearn.pipeline.Pipeline ensures SMOTE is applied only to training folds, preventing data leakage. Next up we explore undersampling techniques that reduce the majority class instead of growing the minority.
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常见问题解答
「随机过采样与 SMOTE」课时是免费的吗？
是的 — 「随机过采样与 SMOTE」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Machine Learning Academy 课程的其余内容，请升级到 CoddyKit PRO。 Machine Learning Academy 课程共包含 4 节课。
「随机过采样与 SMOTE」这节课中我会学到什么？
您将使用 imbalanced-learn 中的 RandomOverSampler 和 SMOTE 对少数类进行过采样，然后评估精确率和召回率是否得到改善。 你通过在浏览器中直接运行的动手代码来练习 Machine Learning Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。
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无需任何先前经验。CoddyKit 上的 Machine Learning Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 2 节课，共 4 节。
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