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基线模型：始终击败 DummyClassifier

您将构建 DummyClassifier 作为最低基线，并确认每个真实模型在被认为有用之前都必须超过该基线

基线模型：始终击败 DummyClassifier 是 CoddyKit 上的免费 Machine Learning Academy 课时。 这是第 4 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 You Need a Baseline

When you report that your model achieves 92% accuracy, that number is meaningless without context. Is 92% impressive or disappointing? It depends entirely on what the simplest possible strategy would achieve on the same problem.

A baseline model is the floor that every real model must beat to be considered useful. Without a baseline, you might celebrate 92% accuracy on a dataset where a model that always predicts 'not fraud' would achieve 95% accuracy — which means your fancy ML model is actually worse than doing nothing.

The DummyClassifier: scikit-learn's Baseline Tool

Scikit-learn provides DummyClassifier as a minimal classifier that makes predictions using simple rules completely ignorant of the input features. It is the formal tool for establishing a baseline before any real modelling begins.

DummyClassifier is not a joke or placeholder — it is a rigorous sanity check. If your real model cannot beat the DummyClassifier, something is fundamentally wrong: your features may not be predictive, your pipeline has a bug, or the problem is harder than expected.

from sklearn.dummy import DummyClassifier
from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split

X, y = load_breast_cancer(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

# Most frequent baseline: always predicts the majority class
dummy = DummyClassifier(strategy='most_frequent')
dummy.fit(X_train, y_train)
baseline_acc = dummy.score(X_test, y_test)
print(f'Baseline accuracy (always predict majority class): {baseline_acc:.3f}')

DummyClassifier Strategies

DummyClassifier supports several baseline strategies:

  • most_frequent: always predicts the class that appears most often in training. Best for imbalanced datasets.
  • stratified: randomly predicts each class with the probability equal to its training frequency. Maintains class distribution.
  • uniform: randomly predicts each class with equal probability. Useful when all classes are equally represented.
  • constant: always predicts a specified constant class. Use to test what happens if you always predict the positive class.
from sklearn.dummy import DummyClassifier
from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split
from sklearn.metrics import f1_score
import numpy as np

X, y = load_breast_cancer(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

for strategy in ['most_frequent', 'stratified', 'uniform', 'prior']:
    dummy = DummyClassifier(strategy=strategy, random_state=42)
    dummy.fit(X_train, y_train)
    acc = dummy.score(X_test, y_test)
    print(f'strategy={strategy}: accuracy={acc:.3f}')

The Accuracy Paradox in Action

The accuracy paradox is most dramatic on imbalanced datasets. Consider credit card fraud where 99% of transactions are legitimate. The most_frequent DummyClassifier predicts 'not fraud' for every transaction and achieves 99% accuracy. A real ML model scoring 97% accuracy would look worse than this baseline by accuracy — even if it correctly identifies most of the actual fraud cases.

This is why the DummyClassifier baseline must be evaluated with the same metric you will use to evaluate your real model. For imbalanced problems, use F1, precision, or recall — not accuracy.

from sklearn.dummy import DummyClassifier
from sklearn.metrics import f1_score, accuracy_score
import numpy as np

# Simulate 99% negative class (not fraud)
np.random.seed(42)
n = 10000
y_true = np.array([0]*9900 + [1]*100)
X_fake = np.random.randn(n, 5)  # random features (not predictive)

# Baseline always predicts not-fraud
dummy = DummyClassifier(strategy='most_frequent')
dummy.fit(X_fake, y_true)
y_pred_dummy = dummy.predict(X_fake)

print(f'Dummy Accuracy: {accuracy_score(y_true, y_pred_dummy):.3f}')  # 0.990
print(f'Dummy F1-score: {f1_score(y_true, y_pred_dummy):.3f}')         # 0.000
print('Dummy catches 0 fraud cases despite 99% accuracy!')

DummyRegressor: Baseline for Regression

For regression problems, scikit-learn provides DummyRegressor with strategies:

  • mean: always predicts the training set mean. This is the standard baseline — R² measures how much better your model is than simply predicting the mean.
  • median: always predicts the training set median. More robust to outliers.
  • quantile: always predicts a specific quantile of the training target.
  • constant: always predicts a specified constant.

A regression model with R² below 0 is actually worse than always predicting the mean — a clear sign something went wrong in training.

from sklearn.dummy import DummyRegressor
from sklearn.linear_model import LinearRegression
from sklearn.metrics import mean_absolute_error, r2_score
from sklearn.datasets import fetch_california_housing
from sklearn.model_selection import train_test_split

X, y = fetch_california_housing(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

dummy_reg = DummyRegressor(strategy='mean')
dummy_reg.fit(X_train, y_train)
y_pred_d = dummy_reg.predict(X_test)

print(f'DummyRegressor MAE: {mean_absolute_error(y_test, y_pred_d):.3f}')
print(f'DummyRegressor R2:  {r2_score(y_test, y_pred_d):.3f}')  # exactly 0.0

Your Model vs the Baseline

Now compare your real model against the baseline using the same metric. The improvement over the baseline tells you how much value your ML model adds. This comparison should be the first result you report in any ML project.

If the improvement is small (e.g., real model achieves F1=0.65 vs baseline F1=0.60), ask whether the complexity and cost of the ML model is worth a 5% improvement. Sometimes a simple rule-based system is cheaper and sufficiently accurate.

from sklearn.dummy import DummyClassifier
from sklearn.linear_model import LogisticRegression
from sklearn.preprocessing import StandardScaler
from sklearn.pipeline import Pipeline
from sklearn.metrics import f1_score
from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split

X, y = load_breast_cancer(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

# Baseline
dummy = DummyClassifier(strategy='most_frequent')
dummy.fit(X_train, y_train)
baseline_f1 = f1_score(y_test, dummy.predict(X_test))

# Real model
pipeline = Pipeline([('scaler', StandardScaler()), ('clf', LogisticRegression(max_iter=1000))])
pipeline.fit(X_train, y_train)
real_f1 = f1_score(y_test, pipeline.predict(X_test))

print(f'Baseline F1: {baseline_f1:.3f}')
print(f'Real model F1: {real_f1:.3f}')
print(f'Improvement over baseline: {real_f1 - baseline_f1:.3f}')

Human-Level Performance as a Second Baseline

For many real-world problems, a human-level performance benchmark is more relevant than a dummy baseline. For example, a radiologist's error rate on detecting tumours, a human spam screener's accuracy, or an expert appraiser's house price estimation error.

Human performance provides a ceiling: if your model matches or exceeds human performance, you have solved the problem. If your model is far below human performance, there is room to grow. If your model is very close to human performance, improving it further may require extraordinary effort for diminishing returns.

Previous System as a Baseline

In industry, the most relevant baseline is usually the existing system your model is replacing. If the current production system uses hand-crafted rules, it is the target to beat. If a previous ML model has been deployed, its production metrics are your baseline.

When reporting results to stakeholders, always compare to the current system, not just a DummyClassifier. Business value comes from improvement over the status quo. A 2% improvement over the current system may be worth millions of dollars in reduced fraud losses, even if it seems small in absolute terms.

Baseline for Multi-Class Classification

For multi-class problems, the DummyClassifier baseline depends on class balance. With K balanced classes, the most_frequent strategy achieves 1/K accuracy. With imbalanced classes, it achieves the proportion of the most frequent class.

For metrics like macro-averaged F1 (which weights all classes equally), a random classifier will score much lower on rare classes. Always compare per-class metrics between your real model and the baseline to understand where the improvement comes from.

from sklearn.dummy import DummyClassifier
from sklearn.metrics import classification_report
from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split

X, y = load_iris(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.3, random_state=42)

dummy = DummyClassifier(strategy='most_frequent')
dummy.fit(X_train, y_train)
y_pred = dummy.predict(X_test)

print('DummyClassifier (most_frequent) on Iris:')
print(classification_report(y_test, y_pred,
      target_names=['setosa', 'versicolor', 'virginica']))
print('Notice: only the majority class has non-zero precision/recall')

Zero-Rule Classifier: The Simplest Baseline

Even simpler than the DummyClassifier is the Zero-Rule (ZeroR) classifier — a common baseline in data science competitions. For classification, ZeroR always predicts the majority class. For regression, it always predicts the mean. ZeroR represents the absolute minimum that any model must beat to be useful.

If you cannot beat ZeroR, your features contain no information about the target variable. This diagnostic is fast and free to compute and should always be the first step in any new ML project before you spend time on complex models.

import numpy as np
from sklearn.datasets import load_breast_cancer
from sklearn.model_selection import train_test_split
from sklearn.metrics import accuracy_score, f1_score

X, y = load_breast_cancer(return_X_y=True)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

# ZeroR: manually compute
majority_class = np.bincount(y_train).argmax()
y_pred_zeror = np.full(len(y_test), majority_class)

print(f'Majority class in training: {majority_class}')
print(f'ZeroR Accuracy: {accuracy_score(y_test, y_pred_zeror):.3f}')
print(f'ZeroR F1 (pos): {f1_score(y_test, y_pred_zeror):.3f}')

Bagging Baselines in Your ML Projects

A complete baseline evaluation workflow for any ML project:

  1. Compute the DummyClassifier (most_frequent) or DummyRegressor (mean) baseline.
  2. If available, compute the current system baseline using its predictions on your test set.
  3. Research human-level performance from benchmarks or literature.
  4. Train your first simple ML model (logistic regression or a shallow decision tree) and compare all three.
  5. Only proceed to complex models if the simple model beats the baseline by a meaningful margin — complexity should be motivated by measurable improvement.
from sklearn.dummy import DummyClassifier
from sklearn.linear_model import LogisticRegression
from sklearn.tree import DecisionTreeClassifier
from sklearn.ensemble import RandomForestClassifier
from sklearn.preprocessing import StandardScaler
from sklearn.pipeline import Pipeline
from sklearn.model_selection import cross_val_score
from sklearn.datasets import load_breast_cancer
import numpy as np

X, y = load_breast_cancer(return_X_y=True)

models = {
    'DummyClassifier': DummyClassifier(strategy='most_frequent'),
    'LogisticRegression': Pipeline([('sc', StandardScaler()), ('clf', LogisticRegression(max_iter=1000))]),
    'DecisionTree(d=5)': DecisionTreeClassifier(max_depth=5),
    'RandomForest': RandomForestClassifier(random_state=42),
}

for name, model in models.items():
    score = cross_val_score(model, X, y, cv=5, scoring='f1').mean()
    print(f'{name}: F1={score:.3f}')

Quick Check

Test your understanding of Machine Learning with Python concepts from this lesson.

Lesson Recap

In this lesson you learned: DummyClassifier establishes the floor of performance that every real model must exceed to provide value, accuracy baselines are misleading for imbalanced datasets — always use the same metric for baseline and real model comparison, and a complete baseline includes the trivial classifier, the current production system, and human-level performance when available. Next up we begin the Data Preprocessing Pipeline course, starting with systematic strategies for handling missing values using imputation techniques.

常见问题解答

「基线模型：始终击败 DummyClassifier」课时是免费的吗？

是的 — 「基线模型：始终击败 DummyClassifier」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。

「基线模型：始终击败 DummyClassifier」这节课中我会学到什么？

您将构建 DummyClassifier 作为最低基线，并确认每个真实模型在被认为有用之前都必须超过该基线 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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「基线模型：始终击败 DummyClassifier」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

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此课程中的所有课时

  1. 为什么不能在训练数据上进行评估
  2. train_test_split：比例、随机种子与分层
  3. 偏差—方差权衡：欠拟合与过拟合
  4. 基线模型：始终击败 DummyClassifier
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