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Answers for “为什么需要特征缩放?”

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Understand why

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Feature scaling is needed because raw features often differ in units and magnitudes, making them incomparable. Scaling brings them onto a common scale, which is necessary for many machine learning algorithms to function properly.

Conditions: Dataset contains multiple features with different units or ranges.; The model is sensitive to feature magnitudes (e.g., distance-based or gradient-based).

Understand why

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Decision-tree models do not require feature scaling because they split data based on feature conditions (thresholds) rather than absolute magnitudes. Monotonic rescaling of feature values does not change the branching structure or the order of splits.

Conditions: Model is a decision tree or an ensemble of trees (random forest, XGBoost, etc.).; Splitting logic is based on comparing feature values to thresholds.

Find a method

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Import `MinMaxScaler` from `sklearn.preprocessing`, instantiate it, and call `fit_transform` on the selected DataFrame columns. This fits the scaler to the data (learning min/max) and transforms the values to the [0, 1] range.

Conditions: Input data is numerical.; Scikit-learn library is installed.

Meet the concept

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Min-max normalization transforms a feature value XX into Xnorm=X−Xmin⁡Xmax⁡−Xmin⁡X_{\text{norm}} = \frac{X - X_{\min}}{X_{\max} - X_{\min}}, mapping the feature's observed range onto the interval [0,1][0, 1].

Conditions: Applied per numerical feature.; Requires a finite minimum Xmin⁡X_{\min} and maximum Xmax⁡X_{\max} for the feature.; Assumes Xmax⁡≠XminX_{\max} \neq X_{min} to avoid division by zero.

Know when to use it

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Feature scaling should be used for algorithms that rely on Euclidean distance or gradient-based optimization, such as KNN, K-means, linear regression, logistic regression, and neural networks. It is generally not needed for tree-based models like decision trees, random forests, and XGBoost.

Conditions: Model uses distance metrics or iterative optimization sensitive to feature scale.; Model is not tree-based or an ensemble of trees.