I used StandardScaler
on the entire dataset before splitting, and later realized that might be wrong. How should I handle scaling correctly to avoid leakage?
joseph1Begginer
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When I first started working with machine learning, I made this classic mistake: I normalized my entire dataset before splitting it. And guess what? My model performed great a little too great. 😅
Turns out, I was leaking information from the test set into training without even realizing it.
Here’s what I do now (and always recommend):
scaler = StandardScaler()
scaler.fit(X_train)
X_train_scaled = scaler.transform(X_train)
X_test_scaled = scaler.transform(X_test)
That way, your model learns only from the training data — just like it would in a real-world setting. No sneak peeks at the test set.
Trust me, once you catch this, you’ll never scale data the old way again. It’s a small thing, but it makes a huge difference in keeping your model honest.
To normalize data without leaking test set information, always follow this golden rule: compute normalization parameters only on the training data.
Here’s the correct process:
Split your data first – before any preprocessing.
Fit the scaler only on training data – e.g.,
Transform both sets using that scaler –
This ensures your model only learns from what it truly should know, preserving the integrity of your evaluation. It’s a small step with a huge impact think of it as respecting the boundary between practice and the real test.