Celine Habashy
Celine Habashy

Reputation: 21

Does the pipeline approach with StandardScaler generalize to tree-based ensembles or neural networks?

I’m using a Pipeline in scikit-learn to combine feature scaling with a classifier. This works well for logistic regression, but I’m curious if this approach would generalize effectively to more complex models like tree-based ensembles or neural networks. Specifically, do these models require different scaling strategies, or can I apply StandardScaler consistently across them?

import numpy as np
from sklearn.model_selection import train_test_split, GridSearchCV
from sklearn.ensemble import RandomForestClassifier
from sklearn.neural_network import MLPClassifier
from sklearn.linear_model import LogisticRegression
from sklearn.preprocessing import StandardScaler
from sklearn.pipeline import Pipeline
from sklearn.metrics import accuracy_score

# Generate sample data
np.random.seed(42)
X = np.random.rand(200, 5)  # 200 samples, 5 features
y = np.random.randint(0, 2, 200)  # Binary target

# Split the data
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

# Define pipelines for different models
pipelines = {
    'logistic_regression': Pipeline([
        ('scaler', StandardScaler()),
        ('classifier', LogisticRegression())
    ]),
    'random_forest': Pipeline([
        ('scaler', StandardScaler()),
        ('classifier', RandomForestClassifier())
    ]),
    'neural_network': Pipeline([
        ('scaler', StandardScaler()),
        ('classifier', MLPClassifier(max_iter=500))
    ])
}

# Evaluate each model
for model_name, pipeline in pipelines.items():
    pipeline.fit(X_train, y_train)
    y_pred = pipeline.predict(X_test)
    print(f"{model_name} Accuracy: {accuracy_score(y_test, y_pred)}")

Upvotes: 0

Views: 26

Answers (1)

chrslg
chrslg

Reputation: 13336

Well, decision trees, and therefore forests, don't really care about scale. They don't perform any computation on values anyway (only on their distribution. I mean, conditional medians, quantiles, etc. But they never add them, multiply them, etc. Except maybe, when using trees/forest for a quantitative regression, to apply some sort of interpolation inside the leaves of the tree, but even then, it is just an interpolation. And anyway, you are using this for binary classification).

So, it doesn't really hurt them to scale the input. Just, it is useless. And since one of the quality of decision trees (less so for random forest) is that they are explainable, you loose that quality, and gain nothing in exchange (if a Decision tree says that, for example, to grant a loan, you can perform classification with if age>70 ⇒ no else if salary > 30000 ⇒ yes else no, that makes sense (you may even be right now thinking that my criteria are strange. Which is probably true, but point is, you can have an opinion on that). If you had scale the data before, the tree would have been identical, but for the scaling, but that would be if age>0.44 ⇒ no else if salary > -0.01 ⇒ else no. Which is less informative.

MLP on another hand, badly need some sort of scaling (not necessarily this one. But something that makes values vaguely between -1 and 1)

Logistic Regression, in theory shouldn't care, but in practice, iterative algorithms can fail if number condition is bad, so it is better to have all data roughly at the same scale.

Anyway, generally speaking, there is a reason why sklearn (or other) don't simply include data scaling in the algorithm. It is because there can't be a general rule about how to scale. Not even for a given algorithm (except that decision trees don't care, that, you can take as a general rule).

For example, nothing says that the "logic" of your data is the same as the distribution of the dataset. You may want to map, in my last example, age between 18 and 100 linearly on -1,1, (with 0 being 59), even tho in your training dataset mean age is 35. You may decide to map not linearly between -1 and 1, but using distribution (so -1 is the min, +1 the max, 0 the median, 0.5 75% percentile, -0.5 25% percentile, etc.). And there again you can do it using the distribution of the dataset, or the distribution in population.

Even more, you may want to use more global scaling, scaling not each feature one by one, but along some axis (for example, using PCA), reduce feature, etc.

So, it doesn't really make sense to try to make a generic pipeline, for any kind of data, and any kind of algorithm, with a predefined scaler. If it did, sklearn would already do it.

Upvotes: 1

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