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    Machine Learning Tutorial 0/98 lessons ~6 min read Lesson 20

    Cross-validation

    What is Cross-validation?

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    Focus
    7 guided sections
    Practice signal
    Examples included
    Career prep
    Foundation builder

    Introduction

    What is Cross-validation? K-fold CV estimates generalization by rotating train/validation splits. Machine learning systems learn patterns from data instead of hard-coded rules.

    Understanding the topic

    How Cross-validation works:

    • K-fold CV estimates generalization by rotating train/validation splits.
    • Prepare or explore data as needed.
    • Train or apply the model/technique.
    • Evaluate results and iterate.
    TermDescription
    Cross-validationK-fold CV estimates generalization by rotating train/validation splits
    Training dataExamples used to learn patterns.
    FeaturesInput variables (columns) fed to the model.
    Target / labelWhat you predict (supervised learning).

    Step-by-step explanation

    1. Understand — Learn when and why to use Cross-validation.
    2. Prepare data — Load, clean, and split datasets.
    3. Apply — Fit model or run algorithm in Python/sklearn.
    4. Evaluate — Measure accuracy, loss, or cluster quality.

    Execution workflow

    1Cross-validation workflow
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    Understand

    Learn when and why to use Cross-validation.

    Best practices

    • Split data into train/validation/test before tuning.
    • Scale numeric features when algorithms are distance-based.
    • Always evaluate on held-out data — not training accuracy alone.

    Common mistakes

    • Training on test data (data leakage).
    • Ignoring class imbalance in classification metrics.
    • Using accuracy alone on imbalanced datasets.

    Summary

    Cross-validation is a core machine learning topic. K-fold CV estimates generalization by rotating train/validation splits

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